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Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California

Christina L. Stamos, Allen H. Christensen, Geoffrey Cromwell, Meghan C. Dick, Christopher P. Ely, Elizabeth R. Jachens · U.S. Geological Survey

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Prepared in cooperation with the Ramona Band of Cahuilla

Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California

Scientific Investigations Report 2025–5073

U.S. Department of the Interior U.S. Geological Survey

Cover. Flowers on a hillside and a well and windmill in Cahuilla Valley. Photographs by Christopher P. Ely, U.S. Geological Survey, 2019.

Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California By Christina L. Stamos, Allen H. Christensen, Geoffrey Cromwell, Meghan C. Dick, Christopher P. Ely, Elizabeth R. Jachens, Sarah E. Ogle, and MacKenzie M. Shepherd

Prepared in cooperation with the Ramona Band of Cahuilla

Scientific Investigations Report 2025–5073

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2025

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Stamos, C.L., Christensen, A.H., Cromwell, G., Dick, M.C., Ely, C.P., Jachens, E.R., Ogle, S.E., and Shepherd, M.M., 2025, Hydrogeologic characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California: U.S. Geological Survey Scientific Investigations Report 2025–5073, 65 p., https://doi.org/10.3133/sir20255073. Associated data for this publication: Ely, C.P., Groover, K.D., Christensen, A.H., and Kohel, C.A., 2020, Electrical resistivity tomography in the Anza-Terwilliger Valley, Riverside County, California 2018: U.S. Geological Survey data release, https://doi.org/10.5066/P9LCEHD7. Fenton, N.C., Christensen, A.H., Shepherd, M.M., and Peterson, M.F., 2020, Select borehole data for Anza Valley, Anza, CA: U.S. Geological Survey data release, https://doi.org/10.5066/P93KA4IG. Shepherd, M.M., Cromwell, G., Ogle, S.E., and Rosenberg, C., 2022, Hydrogeologic data from the Cahuilla Valley and Terwilliger Valley groundwater basins, Riverside County, California, 2022 (ver. 2.0, August 2025): U.S. Geological Survey data release, https://doi.org/10.5066/P9DJLSOV. ISSN 2328-0328 (online)

iii

Acknowledgments This study was funded, in part, by a grant from the California Department of Water Resources, for which the Ramona Band of Cahuilla served as the Local Project Sponsor. The authors thank the Ramona Band of Cahuilla for its assistance in developing the scope of work and for administering the grant funds. The authors are indebted to the well and landowners who provided permission to access their property and collect groundwater-level measurements in their wells. The authors also wish to thank the U.S. Geological Survey current and former field staff, Andrew Morita, Adam Kjos, Greg Smith, Dennis Clark, and Anthony Brown, illustrators Emerson Gusto and Donna Knifong, editor Kelley Calvert, and reviewers for their help in completing this report.

Contents Acknowledgments��iii Abstract��1 Introduction��2 Purpose and Scope��5 Previous Hydrogeologic Studies��5 Accessing Data��6 Description of Study Area��6 Surface Water��7 Land Use��8 Hydrogeology��20 Geologic Setting��20 Groundwater-Bearing Units��23 Alluvium��23 Decomposed and Competent Basement��24 Field Data Collection��24 Groundwater-Level and Precipitation Data��24 Electrical Resistivity Tomography��28 Methods��28 Results��29 Monitoring Wells��31 Geologic Framework Model��33 Well Logs��33 Framework Model Construction��33 Framework Model Results��38 Sources of Recharge��38 Natural Recharge��38 Anthropogenic Recharge��39 Mechanisms of Discharge��40 Evapotranspiration and Evaporation��40 Groundwater Pumpage��41 Groundwater Flow, Levels, and Movement��44 Short-Term Trends in Groundwater Levels��50 Long-Term Trends in Groundwater Levels��54 Summary��59 References Cited��62

Figures 1. Map showing location of study area, including the San Felipe Creek and Santa Margarita River hydrologic subbasins, near Anza, California��3 2. Map showing hydrologic subwatersheds and groundwater basins near Anza, California��4 3. Maps showing land use in the Anza area, California, during 1934, 1945, 1972, 1973, 1986, 1990, 1993, 2001, 2005, 2012, and 2016��9 4. Geologic map showing locations of borehole data from drillers’ logs near Anza, California��21 5. Map showing estimated alluvium thickness near Anza, California��22 6. Graphs showing precipitation data from the Thomas Mountain site and within the Cahuilla Reservation site near Anza, California��28 7. Graphs showing inverted resistivity data for profile 1 and profile 2, near Anza, California��30 8. Diagrams showing well construction information, subsurface lithology, geophysical logs, and groundwater-level data from April 2019 at monitoring sites near Anza, California��32 9. Maps showing thickness of the alluvium, decomposed basement, and the elevation of the top of the competent basement from the geologic framework model near Anza, California��35 10. Cross sections showing the geologic framework model near Anza, California��37 11. Graphs showing estimates of potential maximum evapotranspiration using land use, crop coefficients, and reference evapotranspiration for 1934, 1945, 1972, 1973, 1986, 1990, 1993, 2001, 2005, 2012, and 2016 near Anza, California��42 12. Graph showing estimated annual and cumulative pumpage for 1991–2021 near Anza, California, for substantial water users and domestic users��43 13. Maps showing groundwater-level elevations and contours for 1950, 1973, 1986, and fall 2021 near Anza, California��45 14. Map showing location of wells with short-term hydrographs and precipitation data shown on figure 15 near Anza, California��51 15. Graphs showing groundwater-level hydrographs from wells and precipitation sites near Anza, California��52 16. Map showing location of wells with long-term hydrographs shown on figure 17 near Anza, California��55 17. Graphs showing groundwater-level hydrographs from wells near Anza, California��56

Tables 1. Peak-flow measurements for 1961–73 and 2019 at U.S. Geological Survey site 11042430, near Anza, California��7 2. Wells and precipitation sites in the monitoring network near Anza, California��25 3. Estimates of recharge near Anza, California��39 4. Land-use designations and associated crop coefficients used to calculate evapotranspiration near Anza, California��41

vii

Conversion Factors U.S. customary units to International System of Units

Multiply

To obtain

Length inch (in.)

2.54

centimeter (cm)

inch (in.)

25.4

millimeter (mm)

foot (ft)

0.3048

meter (m)

mile (mi)

1.609

kilometer (km)

Area acre

4,047

square meter (m2)

acre

0.4047

hectare (ha)

acre

0.4047

square hectometer (hm2)

acre

0.004047

square kilometer (km2)

square foot (ft2) square foot (ft2) square mile (mi2) square mile (mi2)

929.0 0.09290 259.0 2.590

square centimeter (cm2) square meter (m2) hectare (ha) square kilometer (km2)

Volume acre-foot (acre-ft)

4,046.9

cubic meter (m3)

gallon (gal)

3.785

liter (L)

gallon (gal)

0.003785

cubic meter (m3)

gallon (gal)

3.785

cubic decimeter (dm3)

Flow rate acre-foot per year (acre-ft/yr)

1,233

cubic meter per year (m3/yr)

acre-foot per year (acre-ft/yr)

0.001233

cubic hectometer per year (hm3/yr)

cubic feet per second (ft3/s)

0.0283

cubic meter per second (m3/sec)

cubic feet per second (ft3/s)

0.0004

cubic meter per day (m3/d)

gallon per day (gal/d)

0.003785

cubic meter per day (m3/d)

gallon per minute (gal/min)

0.06309

liter per second (L/s)

inch per year (in/yr)

25.4

millimeter per year (mm/yr)

Specific capacity gallon per minute per foot ([gal/min]/ft)

0.2070

liter per second per meter ([L/s]/m)

Hydraulic conductivity foot per day (ft/d)

0.3048

meter per day (m/d)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F − 32) / 1.8.

viii

Datums Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88) unless otherwise stated when information is sourced from historical publications. Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Elevation, as used in this report, refers to distance above the vertical datum. Measurements of the distance above or below the elevation datum are referred to as altitude and are referred to as above land surface or below land surface (bls).

Well-Numbering System Wells are identified and numbered according to their location in the rectangular system for the subdivision of public lands. Identification consists of the township number, north or south; the range number, east or west; and the section number. Each section is divided into sixteen 40-acre tracts lettered consecutively (except I and O), beginning with “A” in the northeast corner of the section and progressing in a sinusoidal manner to “R” in the southeast corner. Within the 40-acre tract, wells are sequentially numbered in the order they are inventoried. The final letter refers to the base line and meridian. In California, there are three base lines and meridians: Humboldt (H), Mount Diablo (M), and San Bernardino (S). All wells in the study area are referenced to the San Bernardino baseline and meridian (S). Well numbers consist of 15 characters and follow the format 007S003E29M002S. In this report, well numbers are abbreviated and written 7S/3E-29M2. Wells that are not included in the U.S. Geological Survey (USGS) National Water Information System database are identified using the naming convention used in the original referenced report or non-USGS database.

T1N

TOWNSHIP

T1S T2S T3S T6S T7S

R2W

RANGE R1W R1E

R2E

SECTION 29

R3E

San Bernardino Base Line San Bernardino Meridian

R3W

T7S

7S/3E-29M2

Well-numbering diagram (Note: maps in this report use abbreviated well numbers such as 29M2)

Abbreviations BCM

Basin Characterization Model

bls

Below land surface

CA-BCM

Basin Characterization Model for California

CIMIS

California Irrigation Management Information System

DEM

digital elevation model

DWR

California Department of Water Resources

ERT

electrical resistivity tomography

evapotranspiration

Eto

reference evapotranspiration

GFM

geologic framework model

crop coefficient

milliamp

NHD

National Hydrography Dataset

NWIS

National Water Information System

ohm-m

ohm-meters

PET

potential evapotranspiration

USGS

U.S. Geological Survey

Abstract The relation between the groundwater and the amount of natural recharge to the Cahuilla Valley and Terwilliger Valley groundwater basins is not well understood. During the 20th century, the reliance on groundwater near Anza, California, used for agricultural, domestic, and municipal reasons has increased, and there is the potential for changes in groundwater availability related to climate change. Several types of existing data were evaluated, and new data were collected for this study, with the goal of characterizing the region’s hydrogeology. The study’s scope included constructing a geologic framework model to show where the groundwater-bearing units are present and their relation to each other, estimating the major components of the groundwater budget, and understanding local short-term and regional long-term groundwater flow and how that has changed since the early 1900s. Two electrical resistivity tomography surveys were done in the Durasno Valley about 2,150 feet apart to identify the thickness of the alluvium, its horizontal extent, and the depth-to-basement along two profiles perpendicular to Cahuilla Creek. The subsurface sediments were mostly horizontally layered and the transitional boundary between the alluvium and basement was thinner and shallower along the upgradient profile where the depth-to-basement was about 70 feet below land surface; the depth-to-basement at the downgradient profile was more than about 140 feet below land surface. The results from the surveys were used to place four monitoring wells at two sites along the survey profiles. Artesian flow from the deepest well at the downgradient site indicated that the decomposed and competent basement likely contributed some groundwater to the overlying alluvium, laterally, from below, or both. A digital three-dimensional geologic framework model was constructed using EarthVision software to represent the subsurface geometry of the alluvium, decomposed basement, and competent basement. Maps and cross 1Formerly U.S. Geological Survey.

sections of the modeled thicknesses of the alluvium and decomposed basement, and the modeled elevation of the top of the competent basement, were made to show the subsurface geometry of vertical faults, selected wells, and the groundwater-bearing units. Because natural recharge is related to the variable cycles of precipitation, estimates are difficult to quantify. Recharge and runoff have extreme interannual variability in the study area; recharge and runoff can be sporadic, and a substantive amount may not occur in some years. Estimates of recharge from a previous study and the regional-scale Basin Characterization Model for California for four different periods ranged from 3,800 acre-feet/year for 1897–1947 to 5,900 acre-feet/year for 1971–2000. Potential recharge from the disposal of domestic septic systems may have been as much as 500 acre-feet in 2020. It was estimated that between about 400 and 2,400 acre-feet/year of groundwater is lost through evapotranspiration by vegetation and evaporation from open water bodies, but the main source of discharge is through pumpage, mainly used for agriculture from the alluvium in the Cahuilla Valley and Terwilliger Valley groundwater basins. The estimated total pumpage for 1991–2021 ranged from about 1,140 acre-feet in 2019 to about 3,450 acre-feet in 1994. When summed, the cumulative amount of estimated pumpage between 1991 and 2021 was about 81,400 acre-feet. The general direction of groundwater flow is from the northeast along the San Jacinto fault zone at the headwaters of Cahuilla and Hamilton Creeks, to the surface-water outlets at the west and southeast parts of the study area. Groundwater-level data from the 1950s and earlier indicate that there was a natural groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins, but the changing magnitude and extent of the groundwater depressions caused by pumping since about 1950 indicate that the location of the natural groundwater boundary between the Cahuilla Valley and Terwilliger Valley groundwater basins has migrated over time.

2 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins Flow from the upper to the lower parts of the Cahuilla Valley groundwater basin roughly follows the course of Cahuilla Creek through the narrow Durasno Valley where an estimated volume of flow in April 2019 was about 10–150 acre-feet/year. Short-term trends in groundwater levels, particularly in wells where groundwater is shallow and in the basement unit, show how some areas respond quickly to recharge and discharge. Wells located further to the east within the Cahuilla Valley groundwater basin in the alluvium show much less of a response to recharge events; areas of sustained pumpage from the alluvium, primarily for agriculture, show long-term declines in groundwater levels and generally do not show the effects of storm events or recent runoff. Groundwater levels in wells that are farthest from where most of the recharge occurs and where pumping has been the greatest, had some of the largest long-term groundwater-level declines at a rate of about 0.8 foot/year between 1971 and 2021.

Introduction Groundwater is the sole source of water supply for a rural community and two Native American Tribes in the Anza Valley, California. The relation between the groundwater-bearing units of the surrounding Cahuilla Valley and Terwilliger Valley groundwater basins, and the amount of natural recharge to them, are not well understood. During the 20th century, the reliance on groundwater for agricultural, domestic, and municipal uses has often exceeded recharge, and there is the potential for changes in groundwater availability related to climate change. The

Anza Valley is within the Cahuilla Valley groundwater basin, and rights to pump groundwater within it is adjudicated in U.S.A. v. Fallbrook Public Utility District, and others (Civil No. 51-CV-1247-GPC-RBB; U.S. District Court Southern District of California, 2021). To support the adjudication and management of water resources in the area, the Ramona Band of Cahuilla and the U.S. Geological Survey (USGS) initiated a cooperative study of the hydrogeologic system encompassing the Cahuilla Valley and Terwilliger Valley groundwater basins and parts of the San Felipe Creek and Santa Margarita River hydrologic subbasins (figs. 1, 2). The study area encompasses the rural community in and around the town of Anza and west of Lake Riverside, the southeastern part of the Ramona Band of Cahuilla Reservation, and the Cahuilla Reservation. Residents occupy widely dispersed dwellings throughout the study area, and the population in 2020 was estimated to be about 6,480 (Esri Data Development, 2023). The study area includes the headwaters of the Santa Margarita and San Felipe Creek watersheds, which extend into Riverside, San Diego, and Imperial Counties, California (figs. 1, 2). Two main aquifer units yield groundwater to wells—the alluvium and the underlying competent and decomposed rocks of the basement complex. Previous studies (Moyle, 1976; Woolfenden and Bright, 1988; Landon and others, 2015) identified key gaps in hydrologic data needed to understand the properties of the aquifer system. Improved data coverage and conceptual understanding of the aquifer system presented in this report can be used to monitor the effects of historical and future hydrologic stresses on the Cahuilla Valley and Terwilliger Valley groundwater basins.

Introduction 3 117°30'

117°00'

116°30'

116°00'

N SA

33° 45'

JA CI NT O

Palm Springs

FA UL T Helmet ZO NE

Idyllwild

RIVERSIDE COUNTY

S EA

ta M a San

ita River rgar

Wilso n

Cre e

33° 30'

SA N

Thomas Mountain

ORANGE COUNTY

DR AN

EL SI NO RE FA UL T

Anza

Hami l Creek ton

LT Z

Study area

Map area

ON E

illa Cre e k Cahu

LT O

SAN DIEGO COUNTY

PACIFIC OCEAN

EL SI NO RE FA UL T

Sa n

33° 15'

JA CI NT O

Filipe Creek

FA UL TZ ON E

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

10 10

EXPLANATION National Hydrography Dataset Subbasin

IMPERIAL COUNTY

N SA

Borrego Valley

Study area boundary

San Felipe Creek Santa Margarita

Figure 1. Location of study area, including the Santa Margarita River hydrologic subbasins, near Anza, California.

20 MILES 20 KILOMETERS

4 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins 116°50'

116°45'

116°40'

116°35'

THOMAS MTN PRECIP GAGE NR ANZA CA

33° 35' uilla C ah

333129116481401

Ca hu ill

reek aC

Anza

Ha m

371

ilton SA

C re ek

CIN

Durasno Valley

333132116472401

333348116374101

Creek

11042430

Cahuilla Valley

FA U

LT Z

007S003E34E001S PRECIP

Lake Riverside

33° 30'

Terwilliger Valley

371

e yot k ee

Co Cr

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION National Hydrography Dataset Subwatershed Lower Cahuilla Creek Upper Cahuilla Creek Nance Canyon

Study area boundary

Groundwater basin boundary (California Department of Water Resources, 2020)

Fault—Dashed where approximately located, dotted where concealed

Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation Ramona Band of Cahuilla Reservation

Figure 2. Hydrologic subwatersheds and groundwater basins near Anza, California.

11042430

U.S. Geological Survey crest-stage gage site and identifier Precipitation gage and identifier

THOMAS MTN PRECIP GAGE NR ANZA CA

Purpose and Scope 5

Purpose and Scope The purpose of this long-term, multi-phase study is to characterize the hydrogeology of the Cahuilla Valley and Terwilliger Valley groundwater basins and surrounding groundwater-bearing units, with the ultimate goal of developing a calibrated integrated hydrologic model to help manage the groundwater resources. The purpose of this report is to document (1) the methods and results of field data collection; (2) the geologic framework model developed from well-log reports and regional gravity data; (3) the conceptual understanding of the hydrogeologic system, including sources of recharge and discharge; and (4) the hydrologic stresses and changes in groundwater levels and flow through time. Groundwater and surface-water data have been continuously collected by the USGS since 2017 and the hydrologic framework model was constructed using data from about 580 well logs within the Cahuilla Valley and Terwilliger Valley groundwater basins. Groundwater levels measured in wells from 1916 to 2021 were used to show long-term trends.

Previous Hydrogeologic Studies Many investigations into various aspects of the regional geohydrology in the Anza area have been completed. The following publications are not a complete set of all available reports that were used as references for their historical significance, data reliability, and availability. In addition to groundwater-level data, these initial studies examined well logs and contained descriptions of climate and the local geology that provided a basis for future studies. Early documented groundwater levels in the area surrounding the town of Anza, which Waring (1919) referred to as Babtiste Valley, ranged between 5 and 108 feet (ft) below land surface (bls) in 1916. At that time, wells were no deeper than about 160 ft and primarily were used for homesteads and to grow grain for cattle. Early surveys by the California Department of Water Resources (DWR) documented groundwater levels for many wells in the Anza area as early as 1940, but mainly during the 1950s (California Department of Water Resources, 1956). Two flowing wells were observed in the southwestern part of the Cahuilla Valley groundwater basin in 1953 and 1954, indicating that groundwater hydraulic heads were above the land surface (artesian conditions) in the area west of the Cahuilla Reservation (California Department of Water Resources, 1956). In the late 1940s, farmers began the transition from grain to alfalfa, thus increasing the region’s reliance on groundwater. In fall 1953, the DWR reported 37 active wells producing

from less than 25 gallons per minute (gal/min) to as much as 100 gal/min and specific capacities that ranged from 0.2 to 13.7 gallons/minute/foot (gal/min/ft; California Department of Water Resources, 1956). The DWR compared groundwater-level data from the early 1900s to the mid-1950s in the Anza area and reported that groundwater levels were “slightly, but not appreciably, lower than in 1916” and had declined by about 7 ft (California Department of Water Resources, 1956, p. 73). The documented declines in groundwater levels since the 1950s (California Department of Water Resources, 1956, 1974) initiated more focused studies aimed at aquifer characterization to better quantify the effects of groundwater depletion. Moyle (1976) completed gravity surveys to estimate the thickness of the alluvium, described the general geology of the surface watershed, and collected detailed hydrologic data for the study area. These hydrologic data included summaries of precipitation (1897–1947) with contours of average precipitation between 16 inches (in.) in the lower valley and 30 in. at higher elevations near Thomas Mountain (fig. 2). Woolfenden and Bright (1988) described two main groundwater-bearing units—the alluvium and the weathered and consolidated rock (decomposed and competent basement). Woolfenden and Bright (1988) presented detailed aquifer descriptions based on well logs, differences in specific capacities between wells completed in the alluvium and basement complex, updated precipitation data for the 1940s to the mid-1980s, and compared consumptive use of groundwater between 1973 and 1986. They provided locations of pumping depressions for 1973 and 1986 and patterns of water-quality differences in the study area for surface water and groundwater. The geology within the study area was first compiled by Rogers (1965) in a regional geologic map at 1:250,000 scale. Dibblee and Minch (2008) compiled a geologic map of the northern two-thirds of the study area and most of the groundwater basins at 1:62,500 scale (north of latitude 30° 30’ N) and mapped major faults, including the San Jacinto fault zone, which forms the northeastern boundary of the study area. Surrounding the alluvium are mainly plutonic and metasedimentary rocks. Landon and others (2015) provide detailed analysis of aquifer geometry from well logs and gravity data to estimate the thickness of the alluvium. Maps showing changes in groundwater levels for 2004–13 and 2006–13 also are presented. This report used additional well logs to create the geologic framework model and used groundwater-level data through 2021 for hydrographs and to construct groundwater-level contours.

6 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Accessing Data The groundwater-level and surface-water data presented in this report can be accessed through the USGS National Water Information System Web service (NWISWeb; https://nwis.waterdata.usgs.gov/ca/nwis/nwis; U.S. Geological Survey, 2021). The NWISWeb serves as an interface to a database of site information, including current and historical groundwater, surface-water, and water-quality data collected from locations throughout the United States and elsewhere. Data can be retrieved by state, category, and geographic area and can be selectively refined by specific location or parameter field. NWISWeb can output groundwater-level and water-quality graphs, site maps, and data tables (in Hypertext Markup Language [HTML] and American Standard Code for Information Exchange [ASCII] formats). At the time of this study, there were about 350 sites with groundwater-level measurements from 1916 to 2021 available on NWISWeb for the study area.

Description of Study Area The study area is about 130 square miles (mi2) and is about 20 miles (mi) southwest of Palm Springs (fig. 1). By the end of the 19th century, a small group of the Cahuilla Band had settled in the lowlands of the basin. Today, the Cahuilla Reservation covers about 18,900 acres of the total 82,560 acres of the study area. The Ramona Band of Cahuilla covers about 1,350 acres, of which 755 acres are within the study area. The study area was defined using the National Hydrography Dataset (NHD) to delineate the local drainage basin boundary (U.S. Geological Survey, 2022), which included parts of the Santa Margarita and San Felipe Creek subbasins (fig. 1). Three subwatersheds lie within the study area boundary: the Lower Cahuilla Creek, Upper Cahuilla Creek, and Nance Canyon subwatersheds (Nance Canyon is commonly called the “Terwilliger”; fig. 2). The Upper Cahuilla Creek and Lower Cahuilla Creek subwatersheds have a combined area of about 86 mi2 and drain to the southwest along Hamilton and Cahuilla Creeks; these creeks merge about 2 mi southwest of the town of Anza. Cahuilla Creek continues westward-southwestward into the Santa Margarita River, and ultimately, the Pacific Ocean. The Nance Canyon

subwatershed has an area of about 43 mi2 and is drained by Coyote Creek to the southeast, and ultimately, into the Salton Sea (figs. 1, 2). Hamilton, Cahuilla, and Coyote Creeks are ephemeral, meaning that they have flows of short duration only in response to heavy local precipitation and from infrequent storms and runoff from the surrounding mountains. The Cahuilla Reservation is in the central part of the study area and extends into parts of all three subwatersheds. The study area ranges in elevation from about 6,800 ft near Thomas Mountain, north of Anza, to about 2,100 ft at the point where water in Cahuilla Creek exits the study area (fig. 2). The average annual temperature for Anza is 57 degrees Fahrenheit (°F) with minimum temperatures near freezing in the winter months and maximum temperatures in the 90s °F during summer months (Climate-Data, 2023). Average annual precipitation for the period 1981–2010 ranged from 12.31 in. at the town of Anza to 26.18 in. at Idyllwild, about 15 mi to the north of Thomas Mountain (fig. 1; Western Regional Climate Center, 2020a, 2020b). Snowfall in the Anza area ranges between 10 and 15 in. and generally occurs from early November to late April with a freeze probability of 50 percent or greater between late November and late March (Western Regional Climate Center, 2020a). Precipitation generally occurs in the winter months and from infrequent local thunderstorms in August and September related to monsoon moisture from the south. Storms can vary in intensity from mild to severe; the latter may result in large volumes of precipitation over short periods that cause flash floods and heavy runoff. Within the study area are two groundwater basins, the Cahuilla Valley and Terwilliger Valley (California Department of Water Resources, 2020). These groundwater basins generally are defined by the boundary of the unconsolidated alluvium deposits and the outcrops of the basement complex that surround and underlie the alluvium deposits. The Cahuilla Valley and Terwilliger Valley groundwater basins are much smaller than the surface-water subwatersheds and cover an area of about 29 mi2 and 13 mi2, respectively. Based on groundwater-level data (Moyle, 1976; Woolfenden and Bright, 1988; Landon and others, 2015; U.S. Geological Survey, 2021), a groundwater divide coincides generally with the surface-water boundary between the Upper Cahuilla Creek and Nance Canyon subwatersheds, but the exact location of this divide can migrate depending on the amount of local recharge and discharge (groundwater pumpage) in that area.

Description of Study Area 7

Surface Water

of Water Resources, 1956). Between 1961 and 1973 and in 2019, the USGS measured peak flow along Cahuilla Creek where it crosses State Route 371, west of the town of Anza (U.S. Geological Survey, 2021; fig. 2; table 1); peak flows ranged from 0 to 102 ft3/s. During 2019–20, the USGS installed four crest-stage gages (fig. 2) to estimate peak flows during periodic field visits. Data from the crest-stage gages installed at USGS site 11042430 on Cahuilla Creek and at USGS site 333348116374101 on Hamilton Creek show that these creeks were dry during all visits except one, when flow was estimated by using the indirect methods of Benson and Dalrymple (1967). On February 14, 2019, the estimated discharge into Cahuilla and Hamilton Creeks were 59 ft3/s and 58 ft3/s, respectively. No flow was observed during periodic visits to the other two crest-stage gages west of the Cahuilla Reservation (USGS sites 333129116481401 and 3132116472401). Surface-water discharge records are not available for the many other unnamed ephemeral creeks and washes that converge to form Coyote Creek, which drains out through the southeastern part of Terwilliger Valley (fig. 2).

Surface-water flow occurs mostly in response to precipitation and from snowmelt from the San Jacinto Mountains that drains along ephemeral creeks and small washes into Cahuilla and Coyote Creeks. When present, flow in these creeks passes out of the study area to the southwest via Cahuilla Creek and southeast via Coyote Creek (fig. 2). During wet periods, it is common for ponding water to be present and for evaporation to occur along Cahuilla Creek within and just below Durasno Valley in late winter and early spring, possibly extending into summer owing to episodic run-off events. Historical surface-water discharge measurements in the study area are sparse because of the ephemeral nature of the creeks; therefore, no permanent streamgages have been installed. Despite these challenges, some attempts have been made to estimate surface-water flow when it is present in the study area. Estimates of intermittent discharge in Cahuilla Creek from 1950 to 1954 by the DWR ranged between 0.04 and 1.46 cubic feet per second (ft3/s) and reported a peak flow of about 130 ft3/s in January 1954 (California Department

Table 1. Peak-flow measurements for 1961–73 and 2019 at U.S. Geological Survey site 11042430 (U.S. Geological Survey, 2021) near Anza, California. [—, no data]

Water year

Date

Gage height (feet)

Discharge (cubic feet per second)

Peak streamflow qualification

1961

August 18, 1961

—

102

Discharge is a historic peak.

1962

December 02, 1961

1.41

Discharge is an estimate.

1963

September 03, 1963

1.67

Discharge is an estimate.

1964

April 01, 1964

—

0.2

Discharge is an estimate.

1965

March 07, 1965

—

0.1

Discharge is an estimate.

1966

November 22, 1965

1.68

—

1967

December 06, 1966

1.75

—

1968

December 18, 1967

1.35

—

1969

January 25, 1969

2.06

—

1970

August 16, 1970

1.38

—

1971

—

Month of occurrence is unknown or not exact.

1972

—

Month of occurrence is unknown or not exact.

1973

October 09, 1972

—

0.6

—

2019

February 14, 2019

—

8 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Land Use Much of the undeveloped land outside the Cahuilla Valley and Terwilliger Valley groundwater basins is national forest or state park land. Within the groundwater basins, land has primarily been used for individual homesteads and agriculture. Waring (1919) reported that although several wells existed, no attempt had been made to obtain groundwater for uses other than domestic or stock before 1916. Moyle (1976) reported that about 326 acres of irrigated alfalfa pasture existed in 1953 and that there was a total of about 535 acres of irrigated crops in the eastern section of the Cahuilla Valley groundwater basin. Land use in most of the area is undeveloped with some agricultural use, but rural residential development has increased over time. To quantify land-use change in the study area, spatial data from digital land-use maps were compared for intermittent years when available between 1934 and 2016 (fig. 3). Forty land-use classes were initially categorized based on zoning, some of which were field verified in 11 datasets. Land-use classes were grouped to reduce the number to 12 representative classes (fig. 3). Data for the location of residential property do not exist, so it was assumed that

residential properties are co-located with cropland. Because other land-use classes (urban, cultivated, and industrial) are loosely delineated and lumped together as a single mixed class, the confidence in the spatial extent of the classes is low. From 1934 to 1945, most land development occurred in the northeast part of the Cahuilla Valley groundwater basin and along Cahuilla Creek. By 1972, the land-use data showed increased development (fig. 3C). In 1986, native vegetation accounted for about 91 percent of land use, whereas land with cropland and pasture uses decreased since 1973 (fig. 3E). Land-use changes in 1990 were primarily characterized by an increase in residential land use (fig. 3F); residential land accounted for about 5 percent of total land use, and native vegetation had decreased to about 88 percent. Between 1990 and 1993, land-use changes were minimal, possibly due, at least in part, to a brief economic recession between 1990 and 1991 and an extended period of drought from 1987 to 1992 (fig. 3G). The most significant long-term changes between 1934 and 2016 were in native vegetation and residential use; between 1934 and 2016, approximately 6,900 acres of land were converted to residential use, and 5,600 acres of native vegetation were converted to other uses.

Description of Study Area 9 A

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

CIN

FA U

LT Z

reek aC

Ca hu ill

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1934 land use (Swift and Sabourin, 2000) Cropland and pasture Native vegetation Groundwater basin boundary (California Department of Water Resources, 2020) Cahuilla Valley Terwilliger Valley

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Percentage land use Cropland and pasture (8.9)

Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary

Native vegetation (91.1)

Fault—Dashed where approximately located, dotted where concealed

Figure 3. Land use in the Anza area, California, during A, 1934 (Swift and Sabourin, 2000); B, 1945 (U.S. Bureau of Reclamation, 1996); C, 1972 (U.S. Geological Survey, 1990); D, 1973 (Moyle, 1976); E, 1986 (Woolfenden and Bright, 1988); F, 1990; G, 1993; H, 2001; I, 2005 (1990, 1993, 2001, and 2005 [Southern California Association of Governments, 2005]); J, 2012; and K, 2016 (2012 and 2016 [Southern California Association of Governments, 2019]). Note that the percentage of land use depicted on figures may not add up to 100 because of rounding.

10 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins B

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

CIN

FA U

LT Z

reek aC

Ca hu ill

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1945 land use (U.S. Bureau of Reclamation, 1996) Cropland and pasture Native vegetation Groundwater basin boundary (California Department of Water Resources, 2020) Cahuilla Valley Terwilliger Valley

Figure 3.—Continued

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Percentage land use Cropland and pasture (7.7)

Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Native vegetation (92.3)

Description of Study Area 11 C

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

Ca hu ill

reek aC

CIN

FA U

LT Z

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1972 land use (U.S. Geological Survey, 1990)

Groundwater basin boundary (California Department of Water Resources, 2020)

Commercial

Cahuilla Valley

Cropland and pasture

Terwilliger Valley

Native vegetation Other agriculture Residential Transitional area Vacant land

Figure 3.—Continued

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Percentage land use [May not add to 100 percent because of rounding] Transitional area (3.0) Residential (2.5) Other agriculture (0.2)

Vacant land (0.3) Commercial (0.1) Cropland and pasture (14.2)

Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Native vegetation (79.6)

12 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins D

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

CIN

Ca hu ill

reek aC

FA U

LT Z

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1973 land use (Moyle, 1976) Commercial Cropland and pasture Native vegetation Orchards and vineyards

Groundwater basin boundary (California Department of Water Resources, 2020)

Water (0.2)

Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Residential

Cahuilla Reservation

Water

Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Figure 3.—Continued

Percentage land use

Residential (2.5)

Commercial (0.1)

Orchards and vineyards (0.1)

Cropland and pasture (10.3)

Native vegetation (86.8)

Description of Study Area 13 E

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

CIN

FA U

LT Z

reek aC

Ca hu ill

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1986 land use (Woolfenden and Bright, 1988)

Groundwater basin boundary (California Department of Water Resources, 2020)

Commercial

Cahuilla Valley

Cropland and pasture

Terwilliger Valley

Industrial Native vegetation Orchards and vineyards Residential Utility facilities Water

Figure 3.—Continued

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation

Percentage land use [May not add to 100 percent because of rounding] Water (0.2) Utility facilities (0.1) Residential (2.5) Orchards and vineyards (0.3)

Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Native vegetation (91.1)

Commercial (0.1) Cropland and pasture (5.7) Industrial (0.1)

14 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins F

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

reek aC

CIN

LT Z

Lake Riverside

Ca hu ill

FA U

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1990 land use (Southern California Association of Governments, 2005) Commercial Cropland and pasture

Utility facilities Water Groundwater basin boundary (California Department of Water Resources, 2020)

Percentage land use [May not add to 100 percent because of rounding] Transportation (<0.1) Utility facilities (0.1)

Residential (5.1)

Water (0.2)

Industrial

Cahuilla Valley

Other agriculture (0.5)

Commercial (0.1)

Native vegetation

Terwilliger Valley

Orchards and vineyards (0.4)

Cropland and pasture (5.2)

Open space and recreation (<0.1)

Industrial (<0.1)

Open space and recreation Orchards and vineyards Other agriculture Residential Transportation

Figure 3.—Continued

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Native vegetation (88.5)

Description of Study Area 15 G

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

H am ilto nC S JA

CIN

Durasno Valley

reek aC

FA U

LT Z

Ca hu ill

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 1993 land use (Southern California Association of Governments, 2005) Commercial

Utility facilities Water Groundwater basin boundary (California Department of Water Resources, 2020)

Percentage land use [May not add to 100 percent because of rounding] Transportation (<0.1)

Industrial

Cahuilla Valley

Residential (5.1) Other agriculture (0.5)

Native vegetation

Terwilliger Valley

Open space and recreation (<0.1)

Cropland and pasture

Orchards and vineyards Open space and recreation Other agriculture Residential Transportation

Figure 3.—Continued

Utility facilities (0.1)

Cropland and pasture (5.2) Industrial (<0.1)

Water (0.2) Commercial (0.1)

Orchards and vineyards (0.4) Native vegetation (88.5)

16 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins H

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

reek aC

CIN

FA U

LT Z

Ca hu ill

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 2001 land use (Southern California Association of Governments, 2005) Commercial Cropland and pasture Industrial

Transitional area Utility facilities Water Groundwater basin boundary (California Department of Water Resources, 2020)

Native vegetation

Cahuilla Valley

Orchards and vineyards

Terwilliger Valley

Open space and recreation Other agriculture Residential Transportation

Figure 3.—Continued

Percentage land use [May not add to 100 percent because of rounding] Transportation (< 0.1) Transitional area (< 0.1)

Utility facilities (0.1)

Residential (6.5) Other agriculture (0.6)

Water (0.1) Commercial (0.1)

Open space and recreation (< 0.1)

Cropland and pasture (5.6)

Orchards and vineyards (0.3)

Industrial (0.1)

Native vegetation (86.6)

Description of Study Area 17 I

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

reek aC

CIN

FA U

LT Z

Ca hu ill

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 2005 land use (Southern California Association of Governments, 2005) Commercial Cropland and pasture Industrial

Transitional area Utility facilities

Percentage land use [May not add to 100 percent because of rounding] Transportation (<0.1)

Water Groundwater basin boundary (California Department of Water Resources, 2020)

Transitional area (0.1)

Utility facilities (0.1)

Residential (7.7)

Water (0.1)

Native vegetation

Cahuilla Valley

Other agriculture (0.7)

Orchards and vineyards

Terwilliger Valley

Open space and recreation (<0.1)

Open space and recreation Other agriculture Residential Transportation

Figure 3.—Continued

Commercial (0.1) Cropland and pasture (5.8) Industrial (0.1)

Orchards and vineyards (0.2) Native vegetation (85.0)

18 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins J

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek SA

Durasno Valley

reek aC

CIN

LT Z

Lake Riverside

Ca hu ill

FA U

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 2012 land use (Southern California Association of Governments, 2019) Commercial Cropland and pasture Industrial

Transitional area Utility facilities Water Groundwater basin boundary (California Department of Water Resources, 2020)

Percentage land use [May not add to 100 percent because of rounding] Transportation (<0.1) Transitional area (0.1) Residential (8.3)

Native vegetation

Cahuilla Valley

Other agriculture (0.6)

Orchards and vineyards

Terwilliger Valley

Open space and recreation (<0.1)

Open space and recreation Other agriculture Residential Transportation

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Water (0.2) Commercial (0.5) Cropland and pasture (5.6) Industrial (0.1)

Orchards and vineyards (0.2)

Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Figure 3.—Continued

Utility facilities (0.1)

Native vegetation (84.3)

Description of Study Area 19 K

116°50'

116°45'

116°40'

116°35'

C ah uilla

Cre ek

Thomas Mountain

33° 35'

Anza

Ha mi

l ton

Cr eek

CIN

Durasno Valley

reek aC

FA U

LT Z

Ca hu ill

Lake Riverside

33° 30'

371

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25' 79

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

4 MILES

Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION 2016 land use (Southern California Association of Governments, 2019) Commercial Cropland and pasture Industrial

Transitional area Utility facilities Water Groundwater basin boundary (California Department of Water Resources, 2020)

Native vegetation

Cahuilla Valley

Orchards and vineyards

Terwilliger Valley

Open space and recreation Other agriculture Residential Transportation

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Transportation (<0.1) Transitional area Utility facilities (0.1) (0.1) Residential (8.4) Water (0.2) Other agriculture Commercial (0.5) (0.7) Open space and recreation (<0.1)

Cropland and pasture (5.6)

Orchards and vineyards (0.2)

Industrial (0.1)

Cahuilla Reservation Ramona Band of Cahuilla Reservation Study area boundary Fault—Dashed where approximately located, dotted where concealed

Figure 3.—Continued

Percentage land use [May not add to 100 percent because of rounding]

Native vegetation (84.3)

20 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Hydrogeology Geologic, geophysical, and lithologic data were gathered to describe the hydrogeologic framework of the groundwater basins to help stakeholders get a better understanding of the extent and type of geologic sediments that compose and define the groundwater-bearing units. These data also are critical to help stakeholders understand the controls on groundwater movement. The hydrogeology of the study area was investigated by (1) examining previous studies; (2) evaluating surficial geologic mapping; (3) using geophysical methods to estimate the thickness of the alluvium and depth-to-basement in the Durasno Valley; (4) evaluating drillers’ logs that describe subsurface lithology to construct a hydrologic framework model; (5) determining sources of recharge and discharge; and (6) evaluating groundwater levels and flow.

Geologic Setting The geology of the study area includes Paleozoic and Mesozoic basement complex rocks, unconsolidated to weakly consolidated Neogene to Quaternary sedimentary deposits, and Quaternary surficial sediments (Rogers, 1965; Dibblee and Minch, 2008). The geologic map of Rogers (1965) covers the study area, whereas the more recent and more detailed geologic map of Dibblee and Minch (2008) did not include the southern third of the study area. However, the two maps are in general agreement concerning the spatial distribution of the principal geologic units. These two geologic maps were merged and modified to provide a seamless representation of the study area’s geology (fig. 4). Descriptions of the geologic units on figure 4 and herein are primarily based on the geologic map of Dibblee and Minch (2008). The Neogene to Quaternary sedimentary deposits comprise the nonmarine, semi-lithified Pliocene to Pleistocene Bautista beds (Fraser, 1931; Dibblee and Minch, 2008). These beds, which are also called the Bautista Formation (U.S. Geological Survey and Association of American State Geologists, 2025), have an alluvial sandstone unit and an alluvial-fan gravel and sand unit (fig. 4). The Quaternary surficial sediments consist of weakly indurated Pleistocene (older, surficial sediment) and unconsolidated Holocene (younger, surficial sediment), both of which are comprised of alluvial gravels and sands (Rogers, 1965; Dibblee and Minch, 2008; fig. 4). All these materials comprise the basin-fill sediment of the Cahuilla Valley and Terwilliger Valley groundwater basins. Herein, the materials that comprise the basin-fill sediment are referred to as “alluvium.” The surficial extents of the groundwater basins roughly coincide with the mapped extent of the alluvium (fig. 4). The alluvium is bounded to the north, west, and south by the rocks of the basement complex, which also underlie the groundwater

basins; the alluvium is bounded in the northeast by the San Jacinto fault zone. Coarser-grained alluvial materials are most frequently present at higher elevations near the erosional source rock, and finer-textured materials, including fine sands, silts, and minor clays, are present in the lower elevations. Landon and others (2015) compiled lithologic information from 931 drillers’ logs to determine the areal and vertical extent of the alluvium. The lithologic descriptions of the alluvium were of variable quality, and therefore, it was not possible to reliably determine if the younger surficial sediments were present, or to readily distinguish the contact between the younger surficial sediments and the older surficial sediments, or the Bautista beds (Landon and others, 2015). The thickness and extent of the alluvium is important to determine because these materials contain most of the groundwater in storage in the basins. Well logs compiled by Moyle (1976) showed that the thickness of the alluvium generally ranges from a few ft to about 550 ft; however, there are many places where well logs do not exist or are not available. To investigate the areal and vertical extent of the alluvium, Moyle (1976) completed a relative-gravity survey and determined that, in most places, the alluvium generally is thin and that the rocks of the basement complex are near, or very near, the surface, with the exception of four areas; these areas were confirmed by the isostatic residual gravity field and subsequent model results reported by Landon and others (2015; fig. 5). Both previous gravity studies indicated that the alluvium has an apparent thickness between 200 and more than 1,000 ft along the northeastern boundary of the Cahuilla Valley groundwater basin along the San Jacinto fault zone, and southwest of the Ramona Band of the Cahuilla Reservation. Two other areas where the alluvium is greater than 500 ft thick are the northeastern part of the Cahuilla Reservation in the Cahuilla Valley groundwater basin and the southeastern part of the Cahuilla Reservation in the Terwilliger Valley groundwater basin. The primary rocks of the basement complex are Paleozoic and Mesozoic plutonic, gabbroic, and metasedimentary rocks. The exposed outcrops of the plutonic rocks are mainly gray-white quartz diorite to granodiorite that is massive to slightly gneissic with minor xenoliths (Dibblee and Minch, 2008). Relatively small outcrops of biotite-quartz monzonite, plutonic rocks in the northeast and northwest of the groundwater basins, are often adjacent to similarly small outcrops of gabbroic rocks, which consist of gray-black hornblende diorite to gabbro (Dibblee and Minch, 2008; fig. 4). Metasedimentary rocks are primarily fine-grained, foliated biotite schist that outcrop in the northwestern and southeastern parts of the study. The metasedimentary units can be highly foliated and, where exposed, have a greater degree of diagenesis than the crystalline basement. Small outcrops of quartzite and marble outcrop northeast of the study area.

Hydrogeology 21 116°50'

116°45'

116°40'

116°35' QTf qdi

QTs

hdg

gqm

33° 35'

Qoa

QTs

001

gqm

qdi ms

o n Cr e ek

qdi

207

qdi

062

QTs ms

345

Wil s

hdg

016

B' 082

249

CIN

183

B 416 R rita ive r ga

078

Ma r nta

206

469

578 371

qdi

274

33° 30'

LT Z

Qoa

471

Qa 236

FA U

244 Qa

C' 541

572 Qoa

067

A' qdi

ms qdi RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25'

Qa Qoa ms

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

2 2

Geology modified from Rogers (1965); Dibblee and Minch (2008). Faults modified from California Department of Water Resources (1974); Jennings and others (2010).

4 MILES 4 KILOMETERS

EXPLANATION Geology Surficial sediments Qa—Alluvial sand and gravel of valley areas Older surficial sediments Qoa—Older alluvial gravel and sand, gray Bautista Beds QTs—Alluvial sandstone, light-gray to tan QTf—Alluvial fan gravel and sand Plutonic rocks qdi—Quartz diorite to granodiorite, gray-white gqm—Biotite quartz monzonite, gray-white Gabbroic rocks hdg—Gray-black hornblende diorite to gabbro

Metasedimentary rocks

Study area boundary

ms—Schist, dark-gray, fine grained mq—Quartzite, arkosic, in part recrystallized to plutonic texture or gneissoid quartz diorite ml—Marble (limestone), white to gray-white Groundwater basin boundary (California Department of Water Resources, 2020) Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

Fault—Dashed where approximately located, dotted where concealed

A 067

Line of section Cross-sections well and identifier (Shepherd and others, 2022) Well with borehole data (Fenton and others, 2020) Alluvium Basement and/or decomposed basement Alluvium and decomposed basement

Cahuilla Reservation

Alluvium and basement

Ramona Band of Cahuilla Reservation

Alluvium, decomposed basement, and basement

Figure 4. Surface geologic map and location of wells with drillers’ logs used to interpret borehole data near Anza, California. Modified from Rogers (1965) and Dibblee and Minch (2008).

22 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins 116°50'

116°45'

116°40'

116°35'

THOMAS MTN PRECIP GAGE NR ANZA CA

T. 6 S.

33° 35' uilla C ah

11042430

ilton

Creek

Inset A

Ha m

Anza

T. 7 S.

C re ek SA

007S003E34E001S PRECIP

reek aC

CIN

FA U

LT Z

Ca hu ill

33° 30' 371

Inset A

PROFILE 1

30Q2 30Q3 33° 25'

29M3 C 29M2

la uil

PROFILE 2

T. 8 S.

eek Cr

Durasno Valley

RIVERSIDE COUNTY

SAN DIEGO COUNTY

0 0

.25 .2579

R. 1 E.

.50 MILE

T. 9 S.

.50 KILOMETER

R. 2 E.

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

R. 3 E. 4 MILES

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION Study area boundary

Alluvium thickness, in feet (Landon and others, 2015)

Fault—Dashed where approximately located, dotted where concealed

Less than or equal to 20

U.S. Geological Survey (USGS) surface-water site and identifier

30Q3

Well and identifier

Groundwater basin boundary (California Department of Water Resources, 2020)

Greater than (>) 20 to 50 >50 to 100

Spring (California Department of Water Resources, 1956)

Cahuilla Valley

>100 to 200

USGS National Water Information System spring (U.S. Geological Survey, 2023a)

Terwilliger Valley

>200 to 300

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

>300 to 400

Cahuilla Reservation

>400 to 500

THOMAS MTN PRECIP GAGE NR ANZA CA

Ramona Band of Cahuilla Reservation

>500 to 1,000 >1,000

11042430

D' Electrical Resistivity Tomography profile

PROFILE 1

Figure 5. Estimated alluvium thickness near Anza, California. Modified from Landon and others (2015).

Precipitation gage and identifier

Hydrogeology 23 Basement rocks are described in well-log descriptions (drillers’ logs) and in previous reports as consisting of both a competent (nonweathered) crystalline texture and a highly decomposed (weathered) or fractured texture (Moyle, 1976; Woolfenden and Bright, 1988; Landon and others, 2015; Shepherd and others, 2022). Competent and decomposed basement rock likely underlie the alluvium that comprises the groundwater basins, and it can be difficult to differentiate between the alluvium and decomposed basement rock in the lithologic descriptions in the drillers’ logs. The decomposed basement includes chemically weathered rock, heavily fractured rock, and faulted rock where mechanical weathering from faulting breaks up the basement and may produce fault gouge. In the field, the exposed chemically weathered basement and the faulted decomposed basement were friable and easily removed from an outcrop by hand or with hand tools. Wells are common in areas mapped as basement rock (fig. 4), indicating considerable reliance on groundwater from the decomposed basement in the study area. The transition from decomposed basement to competent basement can be gradual, obscuring the boundary between them, where present. The major Quaternary seismic structure in the study area is the San Jacinto fault zone, which forms the northeastern boundary of the Cahuilla Valley groundwater basin (fig. 4; Jennings and others, 2010; U.S. Geological Survey and California Geological Survey, 2019). The San Jacinto fault zone is part of the primary plate-boundary structure in southern California and is one of the most active strands of the San Andreas fault zone (Onderdonk and others, 2018). As mentioned in the previous section, the alluvium is thickest and the depth-to-basement is greatest adjacent to, and southwest of, the San Jacinto fault zone because the geologic units have dropped relative to those on the northeast side of the fault zone, creating a depression that has been filled by alluvium (Landon and others, 2015). Other unnamed faults of Quaternary and unknown ages mapped by the DWR (California Department of Water Resources, 1974; Jennings and others, 2010; U.S. Geological Survey and California Geological Survey, 2019) are shown on figure 4 as approximately located or concealed. Most faults are southeast-northwest trending, following the strike of the San Jacinto fault zone. Other faults may exist within the study area that have not been mapped or that do not have surface expression, which is possible because of the unusually high seismicity in the area; many swarms of small earthquakes have occurred within the Cahuilla Valley (Sanders and Kanamori, 1984; Earthquake Track, 2024).

Groundwater-Bearing Units The study area has two main groundwater-bearing units that yield groundwater to wells—the alluvium and the underlying competent and decomposed rocks of the basement

complex. Most groundwater pumped from the Cahuilla Valley and Terwilliger Valley groundwater basins is from wells completed in the alluvium within the groundwater basins; however, many more wells have been drilled outside the boundaries of the groundwater basins into the basement complex (fig. 4). Landon and others (2015) noted that about 63 percent of the 931 well logs compiled were drilled into the bedrock/basement complex, which lies outside of the groundwater basins defined by the DWR (California Department of Water Resources, 2020). Lithologic data from other wells that are co-located but completed in the different units indicate that both the basement complex and the alluvium are important sources of groundwater supply, and in some areas, could serve as a connected aquifer system. Landon and others (2015) also found that about 25 percent of the wells compiled penetrated the full thickness of the alluvium and then continued into bedrock and that many wells’ perforations spanned the alluvial deposits and underlying competent and decomposed basement complex. A direct comparison of groundwater-level data between co-located wells completed in both groundwater-bearing units by Landon and others (2015) revealed that there was little variation in groundwater levels between wells completed in the alluvium and basement complex rocks during 1950–2013, indicating that the units are hydraulically connected.

Alluvium The highest yielding production wells in the study area are drilled into the alluvium. Previous investigators (Moyle, 1976; Woolfenden and Bright, 1988) distinguished the younger Holocene alluvium from the older Pleistocene alluvium and Pliocene to Pleistocene Bautista beds; these units were estimated from gravity data to be more than 1,000 ft thick in some places (Landon and others, 2015). Because the younger alluvium is mainly in the stream channels and above the groundwater table, the younger alluvium, older alluvium, and Bautista beds are considered one “alluvium” unit in this report. On a side note, the Interlocutory Judgment No. 33 (Santa Margarita River Watershed Watermaster, 2024a) describes the “shallow aquifer” to include the younger and older alluvial deposits to a maximum but variable depth of about 100 ft. Wells completed in the alluvium are generally in the eastern Cahuilla Valley groundwater basin surrounding the town of Anza and in the central part of the Terwilliger Valley groundwater basin (fig. 5) where the alluvium is relatively thick. Because the younger alluvium is unsaturated in most places, most of the groundwater is from thick sequences of saturated older alluvium (Moyle, 1976). The range of specific capacity values for wells in the alluvium was 1.5–11 gal/min/ft at pumping rates ranging from 200 to 1,100 gal/min (Moyle, 1976; Woolfenden and Bright, 1988).

24 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Decomposed and Competent Basement The basement sourced materials that yield water to wells are moderately to highly decomposed or fractured. Wells open to the basement materials generally have a lower specific capacity and lower production rates than wells open to the alluvium unless the bedrock is highly fractured or decomposed. However, the DWR reported that a large irrigation well drilled in 1954 produced about 1,000 gal/min, and its high yield was notable because it was drilled in “crystalline rocks” (California Department of Water Resources, 1956). Landon and others (2015) reported that fractures of “considerable depth” potentially supply water to many wells completed in bedrock and that the permeable zone of the fractured bedrock extends through the weathered, or decomposed, part of the bedrock and into competent rock. Most of the wells completed in areas mapped as basement material are north of Durasno Valley and west of Anza, but Landon and others (2015) also reported that many wells in the groundwater basins have been drilled into the underlying bedrock where the alluvium is relatively thin. The range of specific capacity values from wells completed in the basement materials was 0.1–2.4 gal/min/ft and reported pumping rates ranged from 5 to 70 gal/min (Moyle, 1976; Woolfenden and Bright, 1988).

Field Data Collection Groundwater, precipitation, and surface-water data were collected during this study to augment existing data that have been collected from previous studies to help better understand the hydrogeology of the groundwater basins and the hydrologic changes through time. Throughout the study area, surface-water and groundwater data have been collected intermittently by the DWR (California Department of Water Resources, 1956) and by the USGS, in cooperation with the State of California (Waring, 1919), the Bureau of Indian Affairs (Moyle, 1976; Woolfenden and Bright, 1988), the High Country Conservancy and Rancho California Water District (Landon and others, 2015), and the Ramona Band of Cahuilla. These historical data, in conjunction with more recent data collected from the current USGS monitoring program, were used to help define the groundwater-bearing units of the aquifer system, to understand how pumpage has affected groundwater levels through time, to document the basin’s response to episodic recharge events (see the “Natural Recharge” section), and to support potential future work, such as regional hydrologic models.

Geophysical data collected for this study, along with data collected from two shallow monitoring-well sites, enabled an in-depth evaluation of the geometry of the Durasno Valley. The characterization of the subsurface structure at this narrow constriction in the middle of the study area provided an opportunity to examine how surface water and groundwater flow from the northeastern to the southwestern parts of the Cahuilla Valley groundwater basin (fig. 5). The two surface electrical resistivity tomography (ERT) surveys that were completed and the two monitoring sites that were installed in the Durasno Valley in 2018 are described in the “Electrical Resistivity Tomography” and “Monitoring Wells” sections.

Groundwater-Level and Precipitation Data Groundwater data have been collected for intermittent studies in the area since about the 1950s (California Department of Water Resources, 1956; Moyle, 1976), but more regular monitoring has occurred since fall 2013 (Woolfenden and Bright, 1988; Landon and others, 2015; U.S. Geological Survey, 2021). Since summer 2017, the USGS has collected groundwater data from selected wells throughout the study area as part of an ongoing monitoring network. Discrete and continuous groundwater-level data that were collected during this study from a total of 90 wells are shown in table 2; the number of wells monitored within the network changes each year depending on well accessibility and integrity, data quality, and wells that were added to replace wells no longer suitable for monitoring. In designing the current monitoring network, wells with historical records were given priority for inclusion to assess groundwater-level changes through time. Many wells with historical groundwater data targeted for inclusion could not be accessed, owing to difficulties in accessing the wells, well failure, or other obstructions. To understand the relation between local recharge and discharge and the effects that these stresses have on groundwater levels, pressure transducers were installed in 18 monitoring wells in the Cahuilla Valley groundwater basin near the town of Anza and within the Durasno Valley (table 2). These transducers collected data at 1-hour intervals and were placed in areas where historical pumpage has been greatest or where rapid responses from recharge events were most likely to occur. Groundwater-level measurements, reported as depth to water in ft bls, were collected by the USGS using calibrated steel or electric tapes in accordance with published USGS technical procedures (Cunningham and Schalk, 2011). All groundwater-level data were quality checked and entered into the USGS National Wate Information System (NWIS; see the “Accessing Data” section; U.S. Geological Survey, 2021).

Hydrogeology 25 Table 2. Wells and precipitation sites in the U.S. Geological Survey monitoring network near Anza, California (U.S. Geological Survey, 2021). [State well numbers based on "Well-Numbering System" section at beginning of this report. Abbreviations: NAVD 88, North American Vertical Datum of 1988; NWIS, National Water Information System (U.S. Geological Survey, 2023); USGS, U.S. Geological Survey; —, no data]

USGS site number

Approximate land-surface elevation (feet above NAVD 88)

Hole depth (feet)

Well depth (feet)

7S/2E-11G5

333441116442801

3,899

—

Discrete water level

7S/2E-13D1

333359116440501

3,787

144

Discrete water level

7S/2E-13M4

333336116435501

3,783

—

Discrete water level

7S/2E-13Q5

333322116433101

3,787

—

Discrete water level

7S/2E-13R1

333319116430701

3,876

137

—

Continuous water level1

7S/2E-14E1

333359116450501

3,766

—

Discrete water level

7S/2E-15A4

333411116451101

3,812

—

Discrete water level

7S/2E-23K1

333243116442201

3,653

—

Discrete water level

7S/2E-24Q2

333228116432801

3,825

—

Discrete water level

7S/2E-24Q3

333238116432601

3,829

—

Discrete water level

7S/2E-32D3

333133116480601

3,380

—

Discrete water level

7S/2E-32J2

333101116472301

3,414

—

Discrete water level

7S/3E-7J1

333428116420801

4,133

—

Discrete water level

7S/3E-7N3

333418116425801

4,018

—

Discrete water level

7S/3E-8H1

333439116411101

4,204

350

Discrete water level

7S/3E-8M1

333430116414501

4,150

—

Discrete water level

7S/3E-8R6

333420116410202

4,041

300

Discrete water level

Abbreviated state well number or atmospheric site name

Type of data collected

7S/3E-9E1

333438116405001

4,231

235

—

Discrete water level

7S/3E-9L4

333433116403201

4,261

—

Discrete water level

7S/3E-9L8

333434116402601

4,261

—

Discrete water level

7S/3E-10B1

333457116391701

4,360

125

Discrete water level

7S/3E-10C1

333459116392801

4,394

—

Discrete water level

7S/3E-11N3

333423116384201

4,132

—

Discrete water level

7S/3E-11N5

333418116385001

4,132

—

Discrete water level

7S/3E-11P3

333418116382801

4,120

—

Discrete water level

7S/3E-13C1

333403116372901

4,207

115

—

Discrete water level

7S/3E-13C2

333359116373501

4,211

—

Continuous water level1

7S/3E-13D1

333358116374201

4,173

Discrete water level

7S/3E-14D1

333357116383401

4,058

—

Discrete water level

7S/3E-14P3

333327116382101

4,032

—

255

Discrete water level

7S/3E-15N2

333323116394301

3,935

106

—

Continuous water level1

7S/3E-15P1

333321116392401

3,939

—

Discrete water level

7S/3E-16H1

333356116395901

4,060

260

Discrete water level

7S/3E-16N5

333321116405501

3,943

150

—

Discrete water level

7S/3E-16P14

333322116404001

3,938

—

Discrete water level

7S/3E-16Q4

333321116402801

3,941

460

Discrete water level

7S/3E-17A5

333405116410901

4,083

—

Discrete water level

7S/3E-17B2

333410116411902

4,088

130

—

Discrete water level

26 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins Table 2. Wells and precipitation sites in the U.S. Geological Survey monitoring network near Anza, California (U.S. Geological Survey, 2021).—Continued [State well numbers based on "Well-Numbering System" section at beginning of this report. Abbreviations: NAVD 88, North American Vertical Datum of 1988; NWIS, National Water Information System (U.S. Geological Survey, 2023); USGS, U.S. Geological Survey; —, no data]

USGS site number

Approximate land-surface elevation (feet above NAVD 88)

Hole depth (feet)

Well depth (feet)

7S/3E-17H1

333351116410201

4,030

136

—

Discrete water level

7S/3E-17H3

333350116405801

4,031

—

Discrete water level

7S/3E-17H4

333352116411301

4,037

—

Discrete water level

7S/3E-17L1

333334116414201

4,032

525

Discrete water level

7S/3E-17P1

333329116413501

4,005

520

Discrete water level

7S/3E-19D2

333310116430501

3,851

—

Discrete water level

7S/3E-19K3

333238116422301

3,986

—

Discrete water level

7S/3E-20A2

333317116411501

3,940

208

205

Continuous water level1

7S/3E-20C2

333315116413101

3,989

551

Discrete water level

7S/3E-21C3

333315116404301

3,924

504

Continuous water level1

7S/3E-21D4

333315116405701

3,935

500

Continuous water level1

7S/3E-21D5

333315116405301

3,930

369

Continuous water level1

7S/3E-21G1

333254116401901

3,866

260

—

Discrete water level

7S/3E-21L1

333244116402801

3,851

Discrete water level

7S/3E-21L3

333240116403201

3,850

117

Discrete water level

7S/3E-21R3

333227116395601

3,860

405

397

Discrete water level

Abbreviated state well number or atmospheric site name

Type of data collected

7S/3E-22D3

333312116394301

3,905

110

—

Discrete water level

7S/3E-22D6

333307116395101

3,887

212

Discrete water level

7S/3E-22G1

333258116392101

3,917

—

Discrete water level

7S/3E-22J2

333251116390001

3,931

—

Discrete water level

7S/3E-23A1

333308116375401

4,160

210

195

Discrete water level

7S/3E-23D1

333313116384601

3,978

—

Continuous water level1

7S/3E-23D3

333314116384601

3,984

555

Continuous water level1

7S/3E-25N1

333135116373901

4,113

—

Discrete water level

7S/3E-27D1

333226116395601

3,850

597

590

Continuous water level1

7S/3E-27D2

333226116395602

3,850

597

375

Continuous water level1

7S/3E-27D3

333226116395603

3,850

597

230

Continuous water level1

7S/3E-27D4

333226116395604

3,850

597

120

Continuous water level1

7S/3E-28D1

333221116404601

3,824

—

Discrete water level

7S/3E-29M2

333156116415201

3,773

Continuous water level1

7S/3E-29M3

333156116415202

3,773

36.5

34.5

Continuous water level1

7S/3E-30Q2

333147116422001

3,733

135

132.5

Continuous water level1

7S/3E-30Q3

333147116422002

3,733

34.5

Continuous water level1

7S/3E-34E1

333122116394001

3,879

249

182

Continuous water level1

7S/3E-34N1

333052116393901

3,962

220

—

Discrete water level

7S/3E-36E2

333119116374801

4,058

238

200

Discrete water level

8S/2E-4N1

332950116471601

3,609

400

Discrete water level

8S/2E-5C3

333035116474601

3,427

195

Discrete water level

Hydrogeology 27 Table 2. Wells and precipitation sites in the U.S. Geological Survey monitoring network near Anza, California (U.S. Geological Survey, 2021).—Continued [State well numbers based on "Well-Numbering System" section at beginning of this report. Abbreviations: NAVD 88, North American Vertical Datum of 1988; NWIS, National Water Information System (U.S. Geological Survey, 2023); USGS, U.S. Geological Survey; —, no data]

USGS site number

Approximate land-surface elevation (feet above NAVD 88)

8S/2E-5G3

333023116473101

3,468

—

Discrete water level

8S/2E-5H1

333022116472501

3,500

225

Discrete water level

8S/2E-5H2

333030116473001

3,497

—

Discrete water level

8S/2E-5K2

333008116473601

3,496

—

Discrete water level

8S/3E-1N3

333002116373901

3,860

—

Discrete water level

8S/3E-2D1

333038116384401

3,882

440

—

Discrete water level

8S/3E-9H1

332941116401401

4,337

—

Discrete water level

8S/3E-11A1

332948116380301

3,864

100

Discrete water level

8S/3E-11M1

332929116384901

3,952

—

Discrete water level

8S/3E-12M1

332927116375601

3,880

—

Discrete water level

8S/3E-12N3

332915116375101

3,782

—

Discrete water level

8S/3E-14D1

332904116390101

4,038

—

Discrete water level

8S/3E-17H1

332850116410501

4,361

—

Discrete water level

8S/4E-7A1

332957116361801

3,862

—

Discrete water level

THOMAS MTN PRECIP GAGE NR ANZA CA

333709116405101

6,815

—

Precipitation2

007S003E34E001S PRECIP

333122116394002

3,879

—

Precipitation2

Abbreviated state well number or atmospheric site name

Hole depth (feet)

Well depth (feet)

Type of data collected

1Data collected at 1-hour intervals. 2Data collected at 15-minute intervals.

Two precipitation sites were installed by the USGS so that the data collected at a higher elevation could be compared to what is received at the lower elevation of the Anza Valley (fig. 5; table 2). Site 007S003E34E001S PRECIP is a climate response atmospheric site where groundwater-level and precipitation data are measured at 15-minute intervals. Another precipitation gage (THOMAS MTN PRECIP GAGE NR ANZA CA) was installed in 2017 to establish a record of local precipitation near Thomas Mountain where data are

collected every 15 minutes. The site at the higher elevation on Thomas Mountain recorded a total cumulative precipitation of more than 48 in. from November 2017 to December 2021 (fig. 6A). Precipitation at the site near the town of Anza (007S003E34E001S PRECIP) recorded a cumulative total precipitation of about 43 in. from September 2017 to December 2021 (fig. 6B), indicating that the higher elevations received about 5 in. more precipitation than the valley over this 39-month period.

28 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins A

B 5

2018

2019

2020

2021

0 2022

0 2017

2018

2019

2020

2021

Cumulative daily precipitation, in inches

Daily precipitation, in inches

Cumulative daily precipitation, in inches

Daily precipitation, in inches

0 2017

50 007S003E34E001S PRECIP

THOMAS MTN PRECIP GAGE NR ANZA CA

0 2022

Calendar year

Calendar year EXPLANATION

EXPLANATION

Daily precipitation

Cumulative daily precipitation

Figure 6. Precipitation data from the A, Thomas Mountain site (USGS 333709116405101) and B, within the Cahuilla Reservation site (USGS 333122116394002) near Anza, California (U.S. Geological Survey, 2021).

Electrical Resistivity Tomography Previous USGS studies of the Cahuilla Valley and Terwilliger Valley groundwater basins defined the thicknesses and characteristics of the aquifer system and identified the location of wells completed in the groundwater-bearing units (Moyle, 1976; Woolfenden and Bright, 1988; Landon and others, 2015); however, the definition and connection between these units in most parts of the study area are not well understood. Surface ERT (also known as direct-current resistivity) surveys provide a reliable method to distinguish between differing subsurface lithology types where sufficient contrast exists between the electrical properties of the deposits. These surveys can also provide valuable estimates for the cross-sectional area of the alluvium and estimates of depth-to-basement. Two ERT surveys were done about 2,150 ft apart to identify the thickness of the alluvium, its horizontal extent, and the depth-to-basement along two profiles perpendicular to Cahuilla Creek across a narrow section of Durasno Valley (fig. 5). The results from the ERT profiles were used to place four monitoring wells at two sites along these profiles in this valley and are discussed in the

“Monitoring Wells” section; the lithology encountered by drilling the two monitoring sites were used to ground truth the ERT results.

Methods In August 2018, an 8-channel SuperSting R8 resistivity/ induced polarization meter (Advanced Geosciences, Inc., 2011) with a maximum of 56 electrodes was used to collect ERT data along two profiles in the Durasno Valley. In this report, the ERT profiles are numbered in downgradient order from the northeast (profile 1) to the southwest (profile 2), which is opposite of how they are numbered in the documentation by Ely and others (2020). Along each profile, a linear array of electrodes spaced at regular intervals was placed in the ground, and several different combinations of current and potential electrode pairs were used to take apparent resistivity measurements. Stainless steel stakes, 18-in. long and approximately 0.5-in. diameter, were driven to a depth of approximately 12 in. Stakes were positioned every 23 ft along upgradient profile 1 (D–D′), which totaled about 1,590 ft in length, and every 16.4 ft along downgradient profile 2 (E–E′), which totaled about 1,120 ft in length (figs. 5, 7). The length of profile 2 was less than profile 1 because of the shorter distance to competent bedrock outcrops at each end of the downgradient profile 2.

Results 29 Information about lateral variability in the subsurface is gained as different combinations of electrode transmitter/ receiver pairs are translated across the array while the instrument reads from a command file. In this same manner, information about greater depths can be obtained by increasing the distance between transmitter and receiver electrodes (Minsley and others, 2010). The transmitted currents are automatically controlled by the resistivity meter depending on the change in voltage between electrodes, which is controlled by the resistivity of the subsurface lithologic units. For profile 1, transmitted currents were between 6 milliamps (mA) and 1,080 mA, with an average current of 611 mA and a standard deviation of 213 mA. Transmitted currents for profile 2 were between 37 mA and 1,159 mA, with an average current of 783 mA and a standard deviation of 195 mA. An inverse Schlumberger array was used for its measurement efficiency and good contrast between lateral and vertical resolution (Minsley and others, 2010). Although the spacing of the electrodes differed slightly between the two profiles, these data were collected using the same array type and inverted using the same methods for interpretation, so the results are comparable. Details about the practical aspects of resistivity surveying techniques can be found in various references (Telford and others, 1990; Binley and Kemna, 2005; Day-Lewis and others, 2008; Milsom and Eriksen, 2011). The resistivity data acquired were processed using the smooth, finite-element inversion method in EarthImager 2D software version 2.4.0, build 617 (Advanced Geosciences, Inc., 2011). The smooth-inversion method finds the smoothest possible model that fits the data to an a priori chi-squared statistic and assumes a Gaussian distribution of data errors. Images of the apparent resistivity data collected are shown in pseudosections, which are a conventional way to present the data but do not represent the true spatial distribution of resistivity values within the earth (Minsley and others, 2010). The pseudosections for the two profiles in Durasno Valley were used as starting models and are documented in Ely and others (2020). Surface topography was incorporated into the inversion to accurately account for electrode geometry and the effect of the irregular earth surface on the distribution of subsurface electric currents. Spatial data for all profiles were collected with the Trimble Geo7x differential global positioning system developed by Trimble, Inc., Sunnyvale,

California. The data were corrected using a University NAVSTAR Consortium geodetic correction (UNAVCO Community, 2005). The inversion was allowed to run a maximum of eight iterations with a stop criterion of 5 percent or less root-mean-square error between the measured and modeled resistivity. If stop criteria were not met, the lowest quality data were considered outliers and removed using a percentage data misfit threshold, chosen to minimize data noise while retaining the maximum amount of data. The inversion was then repeated with the noisiest data removed. For a detailed discussion of the resistivity inverse problem, see Oldenburg and Li (1994) and Binley and Kemna (2005).

Results The interpretation of the two inverted resistivity profiles showed increasing resistivity with depth and mostly horizontally layered lithology (fig. 7). The resistivity of the alluvium was generally between 10 and 75 ohm-meters (ohm-m), represented by the blue and green colors on the profiles. Resistivity of the basement was generally greater than 100 ohm-m and is represented by the orange and red colors on the profiles. The yellow color represents the likely transitional boundary between the alluvium and the decomposed basement. This transitional boundary, which likely represents a zone of decomposed basement, was thinner and shallower along profile 1 (upgradient profile D–D′) than along profile 2 (downgradient profile E–E′); the boundary at profile 2 was deeper and less well defined, indicated by the diffuse colors representing the resistivity values. The depth-to-basement was shallowest in the middle of profile 1, where it was about 70 ft bls, and was greatest in the middle of profile 2, where it was deeper than 140 ft bls. These depth estimates show that the alluvium is shallower and thinner along the upgradient profile where Cahuilla Creek exits the upper part of the Cahuilla Valley groundwater basin and enters Durasno Valley. Based on the approximate area of lower resistivity (represented by the blue and green colors) on the profiles on figure 7, the cross-sectional area of the alluvium is estimated to be about 67,000 square feet (ft2) at profile 1. Only a minimum cross-sectional area of the alluvium of about 62,500 ft2 can be estimated at the downgradient site because the basement unit was not encountered during drilling, and it could not be clearly defined by the ERT data at profile 2.

Elevation, in feet above North American Vertical Datum of 1988

300

400

600

3,550

3,600

Distance, in feet

500

600

PROFILE 2

500

800

700

800

Distance, in feet

700

900

1,000

NORTHWEST E'

1,000

1,100

1,300

Resistivity, in ohm-meters 200.0

1,200

10.3

21.6

45.3

200

300

3,650

100

200

95.0

100

3,700

3,750

3,800

SOUTHEAST E FEET

3,500

3,550

3,600

3,650

3,700

3,750

3,800

7S/3E-29M2 7S/3E-29M3

1,500

Well identifier (Shepherd and others, 2022) Land surface

Well diagram

10.3

21.6

45.3

95.0

Resistivity, in ohm-meters 200.0

EXPLANATION

Well identifier (Shepherd and others, 2022) Land surface

Well diagram

EXPLANATION

1,400

NORTHWEST D'

Figure 7. Inverted resistivity data (Ely and others, 2020) for A, profile 1 (D–D′) and B, profile 2 (E–E′), near Anza, California. Location of section shown on figure 5.

Elevation, in feet above North American Vertical Datum of 1988

FEET 3,850

7S/3E-30Q2 7S/3E-30Q3

PROFILE 1

7S/3E-30Q2

SOUTHEAST D

7S/3E-29M2

30 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Results 31

Monitoring Wells As part of this study, the USGS installed four shallow monitoring wells along Cahuilla Creek in 2018 (figs. 5, 8) using the auger drilling methods described by Shuter and Teasdale (1989). Two wells were installed at each site to better understand the subsurface structure and the vertical and horizontal groundwater gradients in the eastern part of the Durasno Valley. The placement and depth of the monitoring wells were designed to coincide with the location of the ERT profiles. In this narrowest part of the Cahuilla Valley groundwater basin, where it is only about 1,500 ft wide, the volume of groundwater that flows from the Upper Cahuilla Valley through the Durasno Valley and into the lower part of Cahuilla Valley has not been evaluated previously. As part of the drilling and construction of the four monitoring wells, downhole geophysical logs were collected to evaluate the subsurface lithology and to verify the ERT results (Ely and others, 2020; U.S. Geological Survey, 2023). Downhole geophysical logs, a generalized summary of the lithology encountered during the drilling, and the well construction information for the four wells are shown on figure 8. Wells 7S/3E-29M2 and -29M3, which are about 10 ft apart, are in the northeastern part of the Durasno Valley near the upgradient ERT profile 1 (figs. 5, 8B). Wells 7S/3E-30Q2 and -30Q3 are about 15 ft apart near ERT profile 2 (fig. 8A), about 2,150 ft downgradient to the southwest. The land-surface elevation at profile 2 is about 40 ft lower than at profile 1. The lithology encountered during drilling at these sites was primarily sand and silts with interbedded clays. Determining the exact depths where the alluvium was present was difficult because the auger method can smear the materials

when they are brought to the surface for examination. In these situations, the geophysical logs can help determine where predominantly finer-grained sediments—particularly clays—are present. At the upgradient site (wells 7S/3E-29M2 and -29M3; fig. 8B), the geophysical logs indicated that fine-grained sediments were present at about 39 ft bls. Heavily weathered bedrock was encountered at about 68 ft bls, and the auger was not able to penetrate any deeper. Basement (competent or decomposed) was not encountered in the downgradient site (wells 7S/3E-30Q2 and -30Q3; fig. 8A), but the ERT results indicate that the deeper well stopped at a transitional boundary that may have contained some basement rocks of unknown weathering or content, but this could not be definitively determined from the lithology. Groundwater-level data from April 2019 from the wells at both sites showed that the hydraulic heads in the upgradient wells near profile 1 were about 6 ft bls, but the hydraulic heads were very different at the downgradient wells near profile 2 where artesian (flowing) conditions were initially observed in both wells. At the downgradient site, the initial hydraulic head in the deeper well 7S/3E-30Q2 was about 12 ft above land surface and had a higher hydraulic head than the shallower well 7S/3E-30Q3, where groundwater was only slightly above land surface (fig. 8A). Continued groundwater-level monitoring of these sites indicates that artesian conditions have been present in the deep well 7(S/3E-30Q2) since it was installed and that groundwater flows from the well at about 1 gal/min when the well cap is removed. The movement and character of groundwater flow in this valley is discussed further in the “Groundwater Flow, Levels, and Movement” section.

32 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins B Land surface elevation 3,773 feet

–20

Well construction

Summary lithology

600

–10 30Q2

30Q3

is Dista Lan ap nc pro e d su xim betw rfa ate ee ce ly n w 2,1 el 50 ls fee Depth, in feet below land surface t

Natural gamma, Conductivity, in counts in millimhos per second per meter

600

29M2

Summary lithology

29M3

Sand with silty clay Clayey silt with sand

Sand with minor silt and clay

Silty clay

Sand with minor silty clay

Well construction

Sand with clayey silt

Land surface elevation 3,733 feet Silty clay, organic rich

Sand with silty clay

Natural gamma, Conductivity, in counts in millimhos per second per meter

Sand with clayey silt Sand with minor silt, clay, and gravel Sand pack Sand with clayey silt

Perforated interval Heavily weathered bedrock Basement

Depth, in feet below land surface

EXPLANATION Sand with silt and inter-bedded clay

Natural gamma, in counts per second 0 200

Water table

100

110 Sand pack 120 Perforated interval 130

140

Figure 8. Well construction information, subsurface lithology, geophysical logs, and groundwater-level data from April 2019 at monitoring sites A, 7S/3E-30Q2 and -30Q3; and B, 7S/3E-29M2 and -29M3 near Anza, California (U.S. Geological Survey, 2021, 2023).

Results 33

Geologic Framework Model A digital three-dimensional geologic framework model (GFM; Shepherd and others, 2022) was constructed to represent the subsurface geometry of the alluvium, decomposed basement, and competent basement within the study area. This digital model provides a fundamental framework for subsequent studies in the basin. This section describes the compilation of well-log data used for subsurface geologic interpretations, the construction of the GFM using well-log data and other subsurface information, and the results from the GFM.

Well Logs Lithology information from drillers’ log descriptions from about 1,185 well logs (fig. 4; Landon and others, 2015; Fenton and others, 2020) provided information on the subsurface geometry of the alluvium, decomposed basement, and competent basement, and the variability of major lithologic textures within the alluvium. Most well logs were obtained from the DWR Well Completion Report database (California Department of Water Resources, 2022). Some well logs that were not available in the DWR Well Completion Report database were available from hardcopy or digital files of the USGS San Diego Projects office files; these logs were primarily from private water wells drilled more than 50 years ago or on Native American land. Well-test information was also compiled from the well logs where available (Fenton and others, 2020). Summary descriptions of well construction, location (including source and accuracy), regularized lithology information, and well-test data were published by Fenton and others (2020) and Shepherd and others (2022). Drillers’ log descriptions were of variable quality and had a range of detail in the lithologic information (Landon and others, 2015; Fenton and others, 2020; Shepherd and others, 2022). The logs varied in terms of the quantity and thickness of described subsurface lithologic intervals, the detail of the descriptions, and the terminology used to describe sediment types and textures. Quality assurance analysis of the well logs and lithologic descriptions verified the physical location of each well and evaluated the suitability of the lithologic descriptions for understanding the distribution of different lithology types across the study area. Some well logs were removed from consideration because their reported well locations could not be verified or were inconsistent with nearby wells, and some well logs were removed because they had ambiguous or imprecise lithologic descriptions. A final set of 579 well logs and lithologic descriptions was compiled by Shepherd and others (2022) from existing well-log data (Fenton and others, 2020) for use in developing the GFM and lithologic characterization. The drillers’ log descriptions were classified into 54 regularized categories using an approach similar to Faunt (2009) and Landon and others (2015). These regularized lithology categories were used to help interpret the depth

and thickness of geologic units in the GFM (Shepherd and others, 2022). The regularized lithology classes were further simplified into three generalized texture categories that loosely correspond to grain size: (1) coarse gravel, cobbles, boulders, and crystalline rock types; (2) medium sand, sandstone, and decomposed material; and (3) fine clay, silt, mud, and soil. These three textural groups were used to map the distribution of lithology in the alluvium and to show the vertical and spatial distribution of textures in the alluvium along selected sections (see the “Framework Model Construction” section).

Framework Model Construction The GFM was constructed from geologic maps (Rogers, 1965; Dibblee and Minch, 2008), well-log lithologic descriptions (Shepherd and others, 2022), estimated productivity of wells (reported as specific capacity data; Shepherd and others, 2022), and gravity-derived depth-to-basement estimates (Landon and others, 2015). Three geologic units were modeled in the GFM: (1) alluvium, (2) decomposed basement, and (3) competent basement. The alluvium unit is primarily within the Cahuilla Valley and Terwilliger Valley groundwater basins (fig. 4). In the GFM, the alluvium unit was assumed to be present everywhere that it was mapped at land surface and was assumed to fill the subsurface volume between the top of the decomposed basement unit and land surface. The decomposed basement unit is comprised of plutonic or metasedimentary rocks (fig. 4) that have been physically or chemically weathered in situ. Although there may be some areas where basement occurs without the presence of overlying decomposed basement, most well-log lithology descriptions (Shepherd and others, 2022) and field observations indicated that decomposed basement was present in most areas across the study area. Therefore, in the GFM, a relatively thin zone of decomposed basement is assumed to overlie basement everywhere that the competent basement unit is present. The competent basement unit is comprised of plutonic or metasedimentary rocks that have not been physically or chemically weathered. Competent basement rocks are mapped in outcrop and are assumed to underlie the alluvium and decomposed basement. In the GFM, the competent basement unit is assumed to be present throughout the study area. Surface contacts of the geologic units were determined using geologic map information (fig. 4). Subsurface contacts of the three geologic units were determined using well-log lithology descriptions (Shepherd and others, 2022), gravity-derived depth-to-basement estimates (Landon and others, 2015), and estimated well productivity data (Shepherd and others, 2022). Varying quality and detail of well-log lithology descriptions sometimes made subsurface geologic unit interpretations from lithology descriptions difficult to ascertain. Therefore, the gravity-derived depth-to-basement estimates and estimated well productivity data were used as supporting information for interpreting geologic unit contacts for each well log.

34 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins Gravity-derived depth-to-basement estimates corresponded to well-log lithology descriptions of either the decomposed or competent basement in different parts of the study area. Estimates of depth-to-basement from the gravity data corresponded to decomposed basement in most well logs in the Cahuilla Valley groundwater basin, but corresponded to the competent basement in the Terwilliger Valley groundwater basin. Estimated productivity of wells, represented by specific capacity values, was used to help distinguish between the three geologic units. Specific capacity is defined as pumping rate divided by drawdown (unit volume per unit time per unit length; Freeze and Cherry, 1979), and is expressed as gal/min/ft. Following Woolfenden and Bright (1988), specific capacity values greater than 2.4 gal/min/ft were assumed to indicate wells perforated in alluvium, and wells with specific capacity values less than or equal to 2.4 gal/min/ft were generally assumed to be perforated in decomposed or competent basement. The location and type of data used to interpret geologic unit contacts are shown on figure 9. The top of the alluvium unit was interpreted solely from land surface elevation; however, there were wells that encountered the alluvium (fig. 9A), which were used to map the distribution of textures throughout the alluvium with a method described below. Subsurface data for the top of the decomposed basement unit were from well-log lithology data, gravity-derived depth-to-basement estimates, and synthetic control points. Synthetic control points were used to enforce unit geometry and thickness in the absence of appropriate well-log or depth-to-basement data. Subsurface data for the top of the competent basement unit were from well-log lithology data and gravity-derived depth-to-basement estimates. Interpreted geologic unit contacts were used as input data to construct the GFM. Input data for each geologic unit were initially gridded for EarthVision (version 11.0), a three-dimensional geologic-modeling software package that uses a biharmonic cubic-spline interpolation algorithm to produce minimum-tension (minimum-curvature) grids from x, y, z point data (Dynamic Graphics, Inc., 2021). The input data were interpolated to create a grid for the top surface of each unit. The horizontal discretization of the grids was 160 ft in the x (east-west) and y (north-south) directions.

The initial geologic unit grids were exported from EarthVision to a geographic information system (GIS) software (Esri ArcGIS version 10.7.1) and were manually adjusted to ensure that the units’ top grids were consistent with basic geologic principles and the geologic understanding of the Cahuilla Valley and Terwilliger Valley groundwater basins. The units’ top grids were clipped to land surface using a discretized digital elevation model (DEM) grid based on the National Elevation Dataset 10-meter DEM (U.S. Geological Survey, 2019). A minimum thickness of 10 ft was assigned to the alluvium wherever it was interpreted to be present. This value is typically the smallest interval of reported lithologic data from well logs. A minimum thickness of 20 ft was assigned to the decomposed basement within the Cahuilla Valley and Terwilliger Valley groundwater basins wherever the alluvium was present at land surface; this value was chosen because it was generally the smallest thickness of decomposed basement identified in well logs. In areas where plutonic and metasedimentary rocks were mapped at land surface, and in all areas outside of the Cahuilla Valley and Terwilliger Valley groundwater basins (fig. 4), the decomposed basement thickness was set to 75 ft. This value was the mean thickness of decomposed basement from the well-log data (for reference, the median thickness of decomposed basement was 68.5 ft with a standard deviation of 49.9 ft). Lithology data (Shepherd and others, 2022) from well logs shown on figure 9A were interpolated within the alluvium to evaluate the spatial and vertical distribution of lithologic texture classes within that unit. Lithologic data from well logs were characterized into three generalized textural groups (coarse, medium, and fine; see the “Well Logs” section) and interpolated across the alluvium using Rockworks three-dimensional geologic-modeling software (version 20; RockWare, 2021). The interpolation used a lateral blending algorithm that extends lithologic data from a given well and randomizes lithologic correlations within the middle one-third space between adjacent wells (RockWare, 2021). The resulting lithologic interpolation is a solid voxel model of the three generalized texture classes within the alluvium. The lithologic interpolation is continuous between wells in areas with many well logs but is absent in areas with few or no well logs (fig. 10).

Results 35 A.

Thickness of alluvium 116°40' 116° 45'

EXPLANATION Model thickness of alluvium and decomposed basement, in feet

33° 35'

Less than or equal to 50

116° 35'

B' 116° 50'

>250 to 300 >300 to 500

Greater than (>) 50 to 100

>500 to 1,000

>100 to 150

>1,000 to 2,000

>150 to 200

>2,000 to 4,133

>200 to 250

C' 33° 30'

C 0 0

2 2

Study area boundary

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation

4 MILES

Ramona Band of Cahuilla Reservation

4 KILOMETERS

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

Groundwater basin boundary (California Department of Water Resources, 2020)

33° 25'

Cahuilla Valley Terwilliger Valley

Line of section Cross-sections well (Shepherd and others, 2022) Well log (Shepherd and others, 2022)

Figure 9. Thickness of A, the alluvium; B, decomposed basement; and C, the elevation of the top of the competent basement from the geologic framework model (Shepherd and others, 2022) near Anza, California.

36 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins B.

Thickness of decompsed basement 116°40' 116° 45' 33° 35'

116° 35'

116° 50'

EXPLANATION Model thickness of alluvium and decomposed basement, in feet Less than or equal (≤) to 50

33° 30'

0 0

2 2

4 MILES

>250 to 300 >300 to 500

Greater than (>) 50 to 100

>500 to 1,000

>100 to 150

>1,000 to 2,000

>150 to 200

>2,000 to 4,133

>200 to 250

4 KILOMETERS

Modeled altitude (top of competent basement), in feet

33° 25'

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

C. Top of competent basement 116°40'

≤3,000

>4,000 to 4,250

> 3,000 to 3,250

>4,250 to 4,500

>3,250 to 3,500

>4,500 to 5,000

>3,500 to 3,750

>5,000 to 5,500

>3,750 to 4,000

>5,500 to 7,000

Study area boundary

116° 45'

Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

33° 35'

116° 35'

Cahuilla Reservation Ramona Band of Cahuilla Reservation Groundwater basin boundary (California Department of Water Resources, 2020)

116° 50'

Cahuilla Valley Terwilliger Valley Well log (Shepherd and others, 2022)

33° 30'

Control point Gravity-derived depth-to-basement (Landon and others, 2015)

0 0

2 2

4 MILES 4 KILOMETERS

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

Figure 9.—Continued

33° 25'

Results 37 A

NORTH

3,000 0

10,000

20,000

30,000

40,000

BEND IN SECTION 067 60,000

VERTICAL EXAGGERATION ×10

WEST

3,500

249

SECTION A-A'

016

BEND IN 471 SECTION

078

236

HIGHWAY 371

BEND IN SECTION

4,000

BEND IN 082 SECTION

Cahuilla Reservation 4,500

BEND IN SECTION

Cahuilla Valley groundwater basin

345

FEET 5,000

EAST

416

Elevation, in feet above North American Vertical Datum of 1988

SECTION C-C'

50,000

Distance along profile, in feet B

572

BEND IN SECTION 469

274

Cahuilla Reservation

FAULT

3,500

183 BEND IN SECTION

4,000

Cahuilla Reservation

HIGHWAY 371 SECTION B-B'

062

4,500

BEND IN SECTION

001

207

5,000

BEND IN SECTION

Terwilliger Valley goundwater basin

BEND IN SECTION

Cahuilla Valley groundwater basin

SAN JACINTO FAULT ZONE

Elevation, in feet above North American Vertical Datum of 1988

FEET 5,500

SOUTH

3,000 10,000

20,000

30,000

Distance along profile, in feet

EXPLANATION

NORTHEAST

SOUTHWEST

Geology

4,000

206

578

4,500

244 BEND IN SECTION

Cahuilla Reservation

541 BEND IN SECTION

Terwilliger Valley goundwater basin

SECTION 572 A-A'

FEET 5,000

50,000

VERTICAL EXAGGERATION ×10

Alluvium

General texture

Material described in drillers' logs

Coarse

Fine

Gravel, cobbles, boulders, and crystalline rock types Sand, sandstone, and decomposed material Clay, silt, mud, and soil

—

Medium

Decomposed basement Competent basement

3,500 FAULT

Well diagram

3,000 0

10,000

20,000

Distance along profile, in feet

30,000

067

Elevation, in feet above North American Vertical Datum of 1988

40,000

Well identifier (Shepherd and others, 2022) Land surface

VERTICAL EXAGGERATION x10

Figure 10. Sections A, north to south through the Cahuilla Valley groundwater basin; B, west to east through the Cahuilla Valley groundwater basin; and C, southwest to northeast through the Terwilliger Valley groundwater basin from the geologic framework model (Shepherd and others, 2022) near Anza, California. Section lines shown in figures 4 and 9A.

38 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Framework Model Results The GFM provides an improved high-resolution understanding of the hydrogeology in the Cahuilla Valley and Terwilliger Valley groundwater basins. The resulting thickness and extent of the alluvium and decomposed basement, the variability of lithology and grain size within the alluvium, and the structural geometry of the competent basement are discussed in this section. Maps of the modeled thicknesses of the alluvium and decomposed basement and the modeled elevation of the top of the competent basement are shown on figure 9. Sections through the GFM show the geometric relation of the modeled units and vertical faults, along with the depth of selected wells (fig. 10). Additionally, the sections show the interpolated lithologic texture distribution within the alluvium. The alluvium is present throughout much of the Cahuilla Valley and Terwilliger Valley groundwater basins and overlies the decomposed basement. The average thickness of the alluvium is about 140 ft (fig. 10A); this unit is thickest in the Cahuilla Valley and the central part of the Terwilliger Valley groundwater basins, where it is estimated to exceed 450 ft thick, and in the northeastern part of the Cahuilla Valley groundwater basin along the San Jacinto fault zone, where it is estimated to exceed 4,000 ft thick. Estimates of thick alluvium are in areas with limited borehole data and in areas where the tops of the decomposed basement and competent basement are estimated to be relatively deep. The decomposed basement unit is present throughout the study area and underlies the alluvium where alluvium is present; it overlies the competent basement unit everywhere. Decomposed basement is modeled to outcrop everywhere at land surface except where it is overlain by alluvium. Within the groundwater basins, the minimum thickness of the decomposed basement is 20 ft, the maximum thickness is about 230 ft, and the average thickness is about 71 ft (fig. 10B). The competent basement forms the hydraulic base of the study area, underlying the decomposed basement and alluvium (fig. 10C). The elevation of the top of the competent basement follows the general topographic trend of land surface in the study area, whereby the unit is at the highest elevations at the northeastern and southern margins of the study area (elevations greater than 6,700 ft), and at the lowest elevations at the western margin of the study area (elevations less than 3,000 ft). The competent basement forms topographic highs at the boundaries of the Terwilliger Valley and Cahuilla Valley groundwater basins (figs. 9C, 10A). Within the groundwater basins, competent basement forms topographic lows, providing depocenters in which alluvium has accumulated. The most substantial topographic low is in the northern part of the Cahuilla Valley groundwater basin along the southwestern side of the San Jacinto fault zone; there, the top of the competent basement varies more than 6,500 ft between its lowest point in the groundwater basin and its highest point near Thomas Mountain (fig. 10C).

Lithology texture data form the geologic basis for estimating the hydraulic properties that are used in numerical groundwater-flow models (Faunt and others, 2010; Alzraiee and others, 2022). The alluvium is comprised mainly of medium-grained lithologic textures (fig. 10) in most places; the percentage of fine-grained textures is greater in parts of the Terwilliger Valley groundwater basin (figs. 10A, 10C). Coarse-grained textures are present in the alluvium throughout the study area, although typically in small pockets.

Sources of Recharge The sources of natural recharge within the study area include surface-water infiltration from precipitation and runoff that mostly originates in the surrounding mountains and hills. Hamilton Creek and Cahuilla Creek (fig. 5) and the many small unnamed creeks in the study area are ephemeral; no streams from adjacent surface-water basins drain into the study area. When streamflow is present, surface water drains from the basin to the southwest through Cahuilla Creek and to the southeast through Coyote Creek. In addition, there is potential for recharge from septic effluent from domestic septic systems and irrigation return, which is infiltrated water applied to agricultural fields that is not used by plants or lost through evaporation, and reaches the groundwater table.

Natural Recharge The primary sources of natural recharge to the study area are from the infiltration of runoff from the surrounding higher elevations of the San Jacinto Mountains to the northeast and from southeast of the Terwilliger Valley. Recharge also occurs, though in lesser quantities, as surface water from precipitation that is not captured and used by plants (evapotranspiration [ET]). When present, this precipitation can infiltrate the ground surface along the many small ephemeral creeks and washes into the alluvium in the valleys or into the fractures and weathered zones of the basement rocks in upland areas. Recharge and runoff do not occur in the same amounts every year; when very wet years occur, most of the water becomes runoff, and a lesser component becomes recharge. Both recharge and runoff are controlled by the variable cycles of wet and dry weather and are temporally variable across the region (Stern and others, 2021). In addition to being temporally variable, recharge and runoff are spatially variable across the region; recharge frequently occurs outside of the groundwater basins and rarely over the groundwater basins’ footprint (Flint and Flint, 2007). Because natural recharge is related to the variable cycles of precipitation, estimates are difficult to quantify without hydrologic models; however, the previous estimate of recharge by Moyle (1976) and those done as part of this study (Stern and others, 2021) are presented in table 3.

Results 39 Table 3. Estimates of recharge near Anza, California. Time period

Estimated average recharge rate (acre-feet/year)

1897–1947

3,800

Moyle (1976)

1896–2018

4,900

Stern and others (2021)1

1971–2000

5,900

Stern and others (2021)1

1981–2010

4,400

Stern and others (2021)1

Source

1Study area was larger than Moyle (1976).

Moyle (1976) estimated the amount of groundwater recharge based on annual precipitation records for the 50-year period from 1897 to 1947 and assumed that 95 percent of precipitation was used by native vegetation or lost through flood flow in stream channels. The precipitation from that period ranged between 5.21 in. (in 1956) and 22.38 in. (in 1943), and the average annual precipitation ranged from 16 to 30 in. Based on these averages, Moyle estimated that the average annual groundwater recharge was about 3,800 acre-feet/year (acre-ft/year). In arid basins where surface-water data are lacking, the potential in-place recharge and potential runoff into the groundwater system can be estimated using the regional-scale Basin Characterization Model for California (CA-BCM; Flint and others, 2013). The CA-BCM applies a monthly regional water-balance model to simulate hydrologic responses to climate and estimate basin recharge and runoff. The CA-BCM has been used in other desert basins where surface-water data are sparse or nonexistent (Flint and Martin, 2012; Faunt and others, 2015). For the Anza area, stream-flow data from nearby and similar basins, snowpack and the timing of snowmelt, temperature, types of vegetation, and precipitation in the San Jacinto Mountains were used to estimate the average annual potential runoff and recharge. The area that was locally calibrated and validated by Stern and others (2021) was slightly larger than the footprint from Moyle (1976) and encompassed streams and creeks to characterize the water-budget components, including recharge, runoff, stream flow, actual ET, and climatic water deficit. Although some portion of the estimated runoff becomes recharge, the CA-BCM currently does not route water to the groundwater system, and runoff in the CA-BCM is routed only through stream channels; therefore, the amount of runoff that potentially becomes recharge could not be estimated. Recharge and runoff have extreme interannual variability in arid regions, including the study area; the occurrence of recharge and runoff can be sporadic, so the long-term average value for recharge is not a reliable estimate for a particular year in this region (Stern and others, 2021). For years when

precipitation is less than about 8–12 in., the CA-BCM estimated that recharge was negligible, so it is important to note that the estimates in table 3 represent long-term averages, including years when recharge values were zero. The relation of recharge to precipitation is exponential in arid and semi-arid regions and often requires the exceedance of a precipitation threshold to produce substantial recharge or runoff (Stern and others, 2021). This relation is noted because in drier years, the ratio of recharge to runoff is higher, and recharge per unit area can be equal to, or higher than, runoff in some areas. For this area, the CA-BCM estimates for recharge and runoff were highly variable and highly dependent upon which years were used. The variability of recharge and runoff is best illustrated when comparing estimates from different periods. For example, during water years 1896–2018, average monthly recharge ranged from 0 to 11,000 acre-ft (a water year is the period from October 1 to September 30 and is designated by the year in which it ends; for example, water year 2021 was from October 1, 2020, to September 30, 2021). During that period, the average recharge was lower than the average runoff and was estimated to be about 4,900 acre-ft/year (Stern and others, 2021; table 3). During 1981–2010, when dry years occurred more frequently than wet years, the average annual runoff remained about the same, but recharge was estimated to be about 4,400 acre-ft/year. For 1971–2000, which included the highest peak flow on record in 1980, the long-term average recharge was estimated to be about 5,900 acre-ft/year. These examples demonstrate that applying one average annual value of recharge and runoff to this region is not a reliable estimate for any specific year because of the extreme interannual climatic and spatial variability.

Anthropogenic Recharge The town of Anza (fig. 5) and the surrounding community are unincorporated rural areas and therefore rely on private septic disposal systems to dispose of wastewater. The infiltration of water from this source is difficult to quantify, and the volume that reaches the groundwater table likely is small; however, a rough estimate of the potential recharge from private domestic septic systems can be made using an assumed volume of effluent discharged to septic systems and the reported population. Umari and others (1995) used an average septic tank effluent of 70 gallons per day (gal/d) per person to estimate the quantity of septic wastewater that mixed with the underlying groundwater in Victorville, a desert community about 100 mi to the northwest that relies solely on groundwater. Using the 2020 population estimates for the study area of about 6,480 residents (Esri Data Development, 2023), the amount of potential recharge from septic effluent may have been as much as 500 acre-ft.

40 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins Irrigation return is groundwater that has been pumped locally to irrigate agricultural fields but is not consumed by the crops and subsequently infiltrates the groundwater system. The amount of water that is lost through plant use and evaporation and does not return to the groundwater table is considered consumptive use. Estimates of recharge based on crop type, crop efficiencies, and irrigation requirements can be made with accurate estimates of pumpage, percentage of irrigated land, reference ET, and crop coefficients. Estimates of irrigation-return volumes are complicated by many factors, including evaporation and potential significant lag times in arid environments, and are best estimated by hydrologic models that account for variable irrigation efficiencies, soil moisture, the thickness of the unsaturated zone, and complicated crop-irrigation practices and rotation. Stamos and others (2001) estimated irrigation-return flow in the upper part of the Mojave Desert that ranged between 29 and 46 percent of pumpage; however, modern farming methods have increased the efficiency of irrigation, resulting in decreased irrigation-return rates over time.

Mechanisms of Discharge Natural groundwater discharge occurs as ET by vegetation, evaporation from open water bodies, where groundwater is at or near land surface and potentially as underflow where groundwater exits the basins to the west and southeast. Because there is no lithologic or hydrologic boundary separating the Cahuilla Valley and Terwilliger Valley groundwater basins, underflow between them likely occurs, but there are insufficient data to substantiate the direction of underflow at any given time, which is discussed in the “Groundwater Flow, Levels, and Movement” section. In 1953, groundwater discharge from 12 natural springs was reported (California Department of Water Resources, 1956; fig. 5), but field reconnaissance efforts in 2020 did not observe any evidence of groundwater discharge at the reported sites that were accessible. The main source of groundwater discharge is by pumping (extraction) for agricultural, domestic, and municipal uses.

Evapotranspiration and Evaporation Evapotranspiration is the process that removes soil water or groundwater from the subsurface through plant transpiration or direct evaporation. To calibrate the CA-BCM model for estimating recharge (see the “Sources of Recharge” section), Stern and others (2021) used estimates of actual ET from remote sensing and water-balance data for January 2000– December 2013 at 1-kilometer resolution. Estimates of actual ET for four native vegetation types from that study varied greatly and ranged from less than 0.5 in., about 10 millimeters, to about 4 in., or 110 millimeters.

Estimates of potential evapotranspiration (PET), which is the amount of water that would be evapotranspired from an unlimited water supply, were used for this study to determine the maximum amount of water lost through plants. Often, PET is derived using the value for crop coefficient (Kc), a unitless number that represents a crop’s maximum potential water use that varies by plant type, phenology, soil moisture, and crop distribution, multiplied by reference ET (Eto). Defined as the amount of water evapotranspired from well-watered grass of a uniform height, Eto varies based on regional climate patterns (Allen and others, 1998). The California Irrigation Management Information System (CIMIS; California Department of Water Resources, 2012) divides California into zones based on long-term monthly average Eto using data from CIMIS weather stations. Based on the CIMIS weather station data, an Eto of 62.5 inches per year (in/year) was used to determine a course estimate of ET in this area. Estimates of PET rates for this study area for selected years were made by applying Kc values (Allen and others, 1998; Pereira and Alves, 2005; Kjelgren and others, 2016; table 4) to publicly available land-use data for the years 1934 (figs. 3, 11; Swift and Sabourin, 2000), 1945, 1972, 1973 (Moyle, 1976), 1986 (Woolfenden and Bright, 1988), 1990, 1993, 2001, 2005 (Southern California Association of Governments, 2005), 2012, and 2016 (Southern California Association of Governments, 2019). The fourteen different land-use classes and associated Kc values ranged from 0.20 for bare soil to 1.15 for irrigated potatoes and grain. The Kc values for crops represent mid-season single crop classes (Allen and others, 1998; Pereira and Alves, 2013; Kjelgren and others, 2016). Based on the Kc values and the crops present during the selected years shown on figure 3, the maximum PET rates in those years, assuming crops were able to obtain all water required for the entire year, ranged from 24.7 in/year in 1972 to 22.3–21.3 in/year in 2016 (fig. 11). Lower rates in the later years reflected the conversion of native lands to urban developments and agricultural lands. Direct evaporation occurs from the surface of open water and in areas where groundwater is near land surface. Lake Riverside, a man-made reservoir built in 1962 and fed by pumped groundwater, has an area of about 75 acres and is west of the Cahuilla Reservation (fig. 3K). A few much smaller irrigation ponds near the town of Anza, shown as “Water” on figure 3, occasionally are used to temporarily store groundwater when crop demand is greater than the yield of agricultural pumps. Moyle (1976) estimated that the evaporation from these lakes and the reservoir was about 750 acre-ft/year; Woolfenden and Bright (1988) estimated that the evaporation rate was about 740 acre-ft/year in 1973 and about 580 acre-ft/year in 1986. Based on the area of Lake Riverside in 2016, the Kc for “Deep water” (table 4), and the annual evaporation rate of 62.51 in. from CIMIS (California Department of Water Resources, 2012), estimates of the volume of water evaporated from the reservoir was about 390 acre-ft/year.

Results 41 Table 4. Land-use designations and associated crop coefficients (Kc) used to calculate evapotranspiration near Anza, California. Land-use designation

Crop coefficient (Kc; unitless)

Source

Bare soil

0.2

Pereira and Alves, 2005

Conifer trees

Allen and others, 1998

Grain

1.15

Allen and others, 1998

Grass

Allen and others, 1998

Irrigated pasture

0.95

Allen and others, 1998

Deep water

0.65

Allen and others, 1998

Mixed pasture

0.8

Allen and others, 1998

Native vegetation

0.3

Kjelgren and others, 2016

Orchard

0.95

Allen and others, 1998

Potatoes

1.15

Allen and others, 1998

Recreation area

0.45

Calculated by combining 50 percent native vegetation, 25 percent grass, 25 percent bare soil.

Residential

0.375

Calculated by combining 5 percent native vegetation, 25 percent grass, 55 percent bare soil, and 15 percent impervious.

Urban/commercial

0.25

Calculated by combining 75 percent impervious and 25 percent grass.

Shallow water

1.05

Allen and others, 1998

When considering the potential amount of water evaporated from the irrigation ponds, it is important to note that these ponds do not contain water year-round like a reservoir. Assuming that these ponds, which covered an area of about 58 acres in 2016 (fig. 3K), contain water about one-third of the year and have a Kc of 1.05 (see “Shallow water” in table 4), then an estimate of the amount of water evaporated from them was only about 13 acre-ft/year. The combined potential amount of water evaporated from the open water bodies is estimated to be about 400 acre-ft/year. Determining the potential rate of groundwater lost through evaporation where the water table is shallow includes factors that are variable and estimates for this differ widely. Moyle (1976) estimated that the rate of evaporation from groundwater at land surface over 315 acres was about 1,670 acre-ft/yr in 1973, but Woolfenden and Bright (1988) estimated a value that was much larger for 1986, about 3,870 acre-ft/yr from 730 acres. These estimates probably assumed that groundwater was near the land surface over a much larger area than was observed over the course of this study; also, it is likely that the groundwater table was shallower in many places in those years. During this study, evidence of shallow groundwater as saturated terrain and marshy conditions was observed mainly in the area of about 485 acres along Cahuilla Creek, east of Durasno Valley. Assuming a Kc value for pasture, the estimated amount of groundwater lost through evaporation was lower and likely about 2,400 acre-ft/yr at most for wet years and much less for drier years.

Groundwater Pumpage Groundwater has been used as the sole source of water for Native American Tribes, agriculture, and for municipal and domestic supply. Most of the groundwater has been used for agriculture, which likely started near the turn of the 20th century, sometime before 11 irrigation wells were first observed in 1916 by Waring (1919). Pumpage has not been metered, and previous investigators have attempted to estimate the volume of groundwater extracted by pumpage based on land use. In 1953, the DWR inventoried 37 wells (California Department of Water Resources, 1956), and in 1968–69, the DWR compiled information for about 415 unused and active wells, noting that about 535 acres of land were being irrigated (California Department of Water Resources, 1974). Moyle (1976) estimated that, in 1973, 457 acres were being used for irrigated crops and that groundwater was being used at a rate of about 4,200 acre-ft/yr for irrigation, natural pasture, replacement of evaporation from lakes and reservoirs, domestic supply, and livestock. Woolfenden and Bright (1988) reported that, by 1986, there was an increase of about 6,000 acre-ft/yr in consumptive groundwater use since 1973, and the authors estimated that agricultural use increased by 1,820 acre-ft/yr. In comparison, evaporation from natural pasture was estimated to have increased by 2,200 acre-ft/yr and domestic use by 420 acre-ft/yr.

42 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins A

135.0

EXPLANATION Land use

121.5

Potatoes Grain

108.0

Shallow water

Area, in square miles

94.5

Grass Conifer trees

81.0

Orchard

67.5

Irrigated pasture Mixed pasture

54.0

Deep water

40.5

Recreation area Residential

27.0

Native vegetation

13.5

Urban/commercial Bare soil

0 1934

1945

1972

1973

1986

1990

1993

2001

2005

2012

2016

Calendar year B

EXPLANATION

Potential maximum evapotranspiration, in inches per year

Land use Potatoes Grain

Shallow water Grass Conifer trees

Orchard Irrigated pasture Mixed pasture

Deep water Recreation area Residential

Native vegetation Urban/commercial 0

Bare soil 1934

1945

1972

1973

1986

1990

1993

2001

2005

2012

2016

Calendar year

Figure 11. Estimates of A, irrigated area by crop; and B, potential maximum evapotranspiration using land use, crop coefficients, and reference evapotranspiration for 1934 (Swift and Sabourin, 2000), 1945 (U.S. Bureau of Reclamation, 1996), 1972 (U.S. Geological Survey, 1990), 1973 (Moyle, 1976), 1986 (Woolfenden and Bright, 1988), 1990, 1993, 2001, 2005 (Southern California Association of Governments, 2005), 2012, and 2016 (Southern California Association of Governments, 2019) near Anza, California.

Results 43 3,500

90,000

Annual pumpage, in acre-feet

70,000 2,500 60,000 2,000

50,000

1,500

40,000 30,000

1,000 20,000 500

10,000

2021

2020

2019

2018

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

0 1991

Cumulative pumpage, in acre-feet

80,000

3,000

Water year EXPLANATION Agricultural

Cahuilla Reservation

Domestic

Lake Riverside

Municipal

Cumulative

Figure 12. Estimated annual and cumulative pumpage for 1991–2021 near Anza, California, for substantial water users (Santa Margarita River Watershed Watermaster, 2024b) and domestic users.

More recent estimates for “substantial water users” were compiled for 1991–2021 from annual reports by the Santa Margarita River Watershed Watermaster (Santa Margarita River Watershed Watermaster, 2024b; referred to hereafter as the Watermaster) and are shown on figure 12. As defined by the Watermaster, substantial water users are water purveyors, Indian reservations, mobile parks, Lake Riverside (fig. 3K), and private landowners who irrigate 8 or more acres or use the equivalent quantity of water. The Watermaster estimated that the total annual pumpage by substantial water users in the Santa Margarita River watershed for 1991–2021 ranged from about 510 acre-ft in 2019 to about 3,080 acre-ft in 1994. Pumpage from about 1,200 “de minimis” users (those who irrigate less than 8 acres or the equivalent amount of water) were not included in the pumpage estimates by the Watermaster (M. Preszler, Santa Margarita River Watershed Watermaster, written commun., 2023). Pumpage for agriculture was highest between 1991 and 2007—during that time, the average pumpage rate was about 2,030 acre-ft/yr. Pumpage decreased after 2007, and the average agricultural pumpage during 2008–21 was about 1,190 acre-ft/yr (fig. 12). In 2016, 2019, 2020, and 2021, agricultural pumpage was about the same or less than other uses, particularly for Lake Riverside. Although Lake Riverside is considered a substantial water user by the Watermaster,

it does not deliver water to customers, and the groundwater pumped for Lake Riverside is used to maintain the water in the lake that is lost through evaporation. Groundwater used on the Cahuilla Reservation varied between about 40 and 420 acre-ft/yr and was used for grazing of cattle on non-irrigated acreage; it was also used for domestic and commercial uses, which included watering of turf grass, dust control, and for supplying water to a casino (Santa Margarita River Watershed Watermaster, 2024b). The amount of reported pumpage for municipal use was much less than other uses and did not exceed about 50 acre-ft/yr. Domestic pumpage other than on the Cahuilla Reservation was not reported by the Watermaster, so domestic use was estimated from population estimates and per capita water use within the study area boundary. Population data in the study area from Manson and others (2019) for 1990, 2000, and 2010 was about 3,470, 4,550, and 6,110, respectively, and was about 6,480 in 2020 (Esri Data Development, 2023). The population was estimated during each decade by linearly interpreting between years when population data were reported; population estimates for 2021 were assumed to be the same as 2020. The population estimates, excluding the Cahuilla Reservation, were then multiplied by the 2010 per capita water usage of 87 gal/d per person (Dieter and others, 2018) to estimate domestic water use. Based on these data, domestic pumpage was estimated to range between about 340 and 610 acre-ft/yr between 1991 and 2021.

44 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins The estimated total pumpage for 1991–2021 ranged from about 1,140 acre-ft in 2019 to about 3,450 acre-ft in 1994. When summed, the cumulative amount of estimated pumpage between 1991 and 2021 was about 81,400 acre-ft. Although domestic pumpage has been estimated for this report, the amount of pumpage by de minimus users is unknown, and their use may be significant in some years (M. Preszler, Santa Margarita River Watershed Watermaster, written commun., 2023); therefore, the actual annual pumpage volumes were higher than those shown on figure 12.

Groundwater Flow, Levels, and Movement The general direction of groundwater flow is from the northeast along the San Jacinto fault zone and headwaters of Cahuilla and Hamilton Creeks, to the surface-water outlets at the western and southeastern parts of the study area. From the higher elevations, groundwater generally flows southwestward through the Cahuilla Valley, following the general course of Cahuilla and Hamilton Creeks. Groundwater elevations are highest in the basement rocks near the fault zone, and previous investigators reported groundwater elevations higher than 4,700 ft above mean sea level and depths to groundwater as much as 340 ft bls (fig. 13). The lowest groundwater elevations are to the west where Cahuilla Creek exits the study area at about 3,400 ft. Within the Cahuilla Valley groundwater basin, depth to groundwater is greatest southwest of the San Jacinto fault zone and is generally near or above land surface in the middle and southwestern parts of the basin. Maps depicting the groundwater elevations and flow directions at selected periods help describe the changing hydrology of the groundwater basins and document how recharge (precipitation and runoff) and discharge (evapotranspiration, evaporation, and pumpage) affect the aquifer system (fig. 13). Moyle (1976) collated available groundwater-level data from 1950 and produced a contour map of the groundwater-level elevations, which he presented as steady state to represent conditions that existed before large-scale pumping had a significant effect on groundwater levels (fig. 13A). This representation of steady state in 1950 is reasonable in most places, considering that the DWR inventoried only 37 wells in 1953 (California Department of Water Resources, 1956); however, some areas had already been affected by pumpage, such as the area within and northeast of the town of Anza where groundwater-level contours show irregularities, including abrupt curves and steep gradients. The contoured data from 1950 also indicate that there was a groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins and that water that entered on the northeastern side of the surface-water divide flowed towards the Cahuilla Valley, and water that entered on the southeastern side flowed toward the Terwilliger Valley groundwater basin (fig. 13A).

Moyle’s groundwater-level contours extend beyond the boundaries of the Cahuilla Valley and Terwilliger Valley groundwater basins that are currently defined by the DWR (California Department of Water Resources, 2020) and cover the surrounding areas of exposed basement, hydraulically connecting the alluvium and the decomposed and competent basement (figs. 13A, 13B). Woolfenden and Bright (1988) examined the groundwater-level data from 1973 and 1986 from wells completed within the competent basement and alluvium (figs. 13B, 13C) and concluded that the hydraulic head did not vary substantially with depth, supporting the suggestion by Moyle that the groundwater-bearing units are hydraulically connected. Moyle (1976) compared data for 1950 and 1973 and observed that pumping had changed the pattern of groundwater flow in some places in the 23 intervening years. Groundwater-level contours from 1973 by Moyle (1976) and modified by Woolfenden and Bright (1988; fig. 13B) showed that the overall regional direction of groundwater flow had not changed; however, pumpage had affected groundwater flow locally, and there were several areas of groundwater-level depressions in both the Cahuilla Valley and Terwilliger Valley groundwater basins, particularly in the area north of the Cahuilla Reservation. Woolfenden and Bright (1988) noted further perturbances to the groundwater-flow patterns in 1986 where continued decreases in groundwater levels resulted in larger pumping depressions next to the San Jacinto fault zone in the northeast, between Cahuilla Creek and Hamilton Creek, and southeast of the Cahuilla Reservation in the Terwilliger Valley (fig. 13C). These depressions were in areas where the alluvium is thickest and near wells that are capable of higher pumping rates compared to wells in other parts of the area. Through time, these groundwater-level depressions have migrated slightly in location and fluctuated in depth depending on how much and where groundwater was extracted for irrigation, and the amount of rainfall and subsequent recharge that reached the groundwater system each year. Groundwater-level elevations and contours for wells measured in fall 2021 are shown on figure 13D. Groundwater-level contours were not constructed outside of the Cahuilla Valley and Terwilliger Valley groundwater basins, but discrete groundwater-level data for 2021 outside of the basins are included to show the similarities in groundwater levels between the DWR-defined groundwater basins and areas where the basement crops out at land surface. The contours of the 2021 data show that pumping continued to affect groundwater levels in the Cahuilla Valley groundwater basin near the town of Anza and north of the Cahuilla Reservation. The variability and extent of the groundwater elevations in the area over time indicate that the location of the natural groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins migrates depending on hydrologic stresses (figs. 13A–D). Data from wells that are closer to the boundary of these two groundwater basins would clarify groundwater flow and the location of the groundwater boundary under current conditions.

Results 45 A

116°50'

116°45'

116°40'

Ja ci n

116°35' T. 6 S.

ai n

33° 35'

Valley

4,50

Anza

3,9

H am ilto n

CIN

FA U

LT Z

4,000

3,850

3,900

3,9

371

3,8

Terwilliger Valley

4,100

3,70 0

3,60

3,500

3,400

Ca hu illa

33° 30'

ek Cre

T. 7 S.

4,500

Creek

4,100

Cah u il

3,8

4,000 Cahuilla

e yot k ee

Co Cr

T. 8 S.

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25'

T. 9 S.

R. 1 E. R. 2 E. Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

R. 3 E. 4 MILES

2 2

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION Groundwater basin boundary (California Department of Water Resources, 2020) Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation

Study area boundary 4,

100

1950 groundwater elevation contour, in feet (Moyle, 1976)—Dashed where approximately located; contour interval is variable; datum is National Geodetic Vertical Datum of 1929 Fault—Dashed where approximately located, dotted where concealed

Ramona Band of Cahuilla Reservation

Figure 13. Groundwater-level elevations and contours for A, 1950 (Moyle, 1976); B, 1973 (Woolfenden and Bright, 1988); C, 1986 (Woolfenden and Bright, 1988); and D, fall 2021 (U.S. Geological Survey, 2021) near Anza, California.

46 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins B

116°50'

116°45'

116°40'

116°35'

4,0

4,15

hu il l

33° 35'

Creek 4,0 4,0 00 00

T. 6 S.

0 3,95 0 3,80

CIN

FA U

LT Z

3,4

50 3,55

Terwilliger Valley

3,82

3,825 00 3,8

4,100

33° 30'

3,850

Ca hu ill

T. 7 S.

eek

4,2

3,8

4,70

3,800

3,75

l ton

4 ,0

reek aC

Ha mi

00 3,9

3,700

3,9 50 Cahuilla Anza Valley

371

e yot k ee

Co Cr

T. 8 S.

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25'

T. 9 S.

R. 1 E. R. 2 E. Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

R. 3 E. 4 MILES

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

Figure 13.—Continued

Study area boundary 4,

100

1973 groundwater elevation contour, in feet (Woolfenden and Bright, 1988)—Dashed where approximately located; hachures indicate depression; contour interval is variable; datum is National Geodetic Vertical Datum of 1929 Fault—Dashed where approximately located, dotted where concealed

Results 47 C

116°50'

116°45'

116°40'

116°35'

4,1 00 00

4,5 00

ll a

4 ,0

4,1

CIN

3,950 85 3,8

FA U

LT Z

Valley

3,850

3,8

3,82

4,2

4,20

371

25 Terwilliger

3,9

3,8

3,5

3,6

Ca hu ill

3,850

T. 7 S.

ek C re N

4,2

3,8

3,4 0 5 3,4

3,8

33° 30'

ilto n

3,7

reek aC

Ha m

3 ,9

Creek

3,7 00

00 3,9

C ah u i

4,000

Anza Cahuilla Valley

4,3

3,80

4 ,0

4,000

4,1 50 4,10 0

00 3,9 50 3,8

33° 35'

T. 6 S.

e yot k ee

Co Cr

T. 8 S.

RIVERSIDE COUNTY SAN DIEGO COUNTY 33° 25'

T. 9 S.

R. 1 E. R. 2 E. Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

R. 3 E. 4 MILES

2 2

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

Figure 13.—Continued

Study area boundary 4,

100

1986 groundwater elevation contour, in feet (Woolfenden and Bright, 1988)—Dashed where approximately located; hachures indicate depression; contour interval is variable; datum is National Geodetic Vertical Datum of 1929 Fault—Dashed where approximately located, dotted where concealed

48 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins 116°50'

116°45'

116°40'

17H1 3,924

116°35' T. 6 S.

11N3 3,783 10C1 4,376 10B1 11N5 3,781 4,329

17B2 74 4,009 8H1 13C1 9L8 11P3 11G5 4,162 8M1 4,002 3,997 3,781 3,788 17A5 4,074 13C2 3,982 7N3 14E1 13D1 17H3 4,168 3,871 14D1 4,0 3,750 3,753 3,904 3,790 0 Cahuilla 0 22D3 Valley 17L1 4,000 ? 17P1 3,874 3,888 H 0 3 0 ,9 ami 3,868 l t on 20C2 14P3 Cr Anza 22D6 3,894 24Q3 3,920 20A2 23D3 e ek 3,700 ? 21D5 3,737 3,863 ? 3,845 3,909 ? 3,848 SA 15P1 N 22J2 3,905 23D1 21L1 27D4 JA 24Q2 3,909 CIN 3,901 3,814 3,809 3,714 TO FA 21L3 30Q3 UL 3,815 TZ 25N1 3,729 29M3 3,763 ON 3,981 34E1 E 21D4 21C3 3,814 3,856 3,823 Terwilliger 7A1 Valley 21G1 1N3 3,803 11A1 3,809 3,808 3,806 11M1 3,804 12M1 3,806 14D1 ? 0 3,807 17H1 3,80 4,277

3,80 0

3,9

33° 35'

16P14 3,824

Cah u ? i l l a Creek

4,0

00 ?

3,7

371

? 3,90

4N1 3,439

? 3,850

5H1 3,408

? 3,800

5G3 3,413

? ?

,60

ah uil la

5C3 3,406 5K2 3,426

? ?

33° 30'

32J2 3,401

eek Cr

3,7

3,5

T. 7 S.

Coyote Creek

T. 8 S.

RIVERSIDE COUNTY SAN DIEGO COUNTY

33° 25'

T. 9 S.

R. 1 E. R. 2 E. Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

0 0

R. 3 E. 4 MILES

2 2

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION Groundwater basin boundary (California Department of Water Resources, 2020)

100

Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation Ramona Band of Cahuilla Reservation

Study area boundary

Figure 13.—Continued

2021 Groundwater-level elevation contour, in feet— Dashed where approximately located; hachures point into depression; contour interval is variable; datum is North American Datum of 1988 (NAVD 88) Fault—Dashed where approximately located, dotted where concealed

4N1 3,439

Well and groundwater-level altitude (2021)—Top text is abbreviated state well number; bottom text is groundwater-level altitude (NAVD 88)

Results 49 Groundwater data from structurally complex areas in the Cahuilla Valley groundwater basin point to mechanisms other than recharge and discharge that affect groundwater flow. The effects on groundwater flow on a large scale are evident in the northeastern part of the basin, where the strands of the San Jacinto fault zone act as barriers, or partial barriers, to groundwater flow (Landon and others, 2015). The fault’s effects are evident in the data from wells adjacent to, and within, this fault zone; these wells are less than 1 mi from each other but have differences in groundwater-level elevations of hundreds of ft. In April 2021, almost 390 vertical ft separated the groundwater-level elevations in well 7S/3E-13C2 (4,168 ft) on the east side of the fault zone, where depth to groundwater was about 42 ft bls, and well 7S/3E-11P3 (3,781 ft) on the west side of the fault zone, where depth to groundwater was about 340 ft bls (fig. 13D). In this zone, the fault strands act as barriers to groundwater flow likely owing to the cementation from mineral deposition within the fault zone and deformation of the geologic and groundwater-bearing units resulting from seismic activity along the fault (Londquist and Martin, 1991; Stamos and others, 2001). Commonly referred to as fault gouge, the fine-textured sediment or rock that fills fault planes has been observed in the San Jacinto fault zone by Dor and others (2006). On a smaller scale, data collected from the Durasno Valley and discussed in the “Electrical Resistivity Tomography” and “Monitoring Wells” sections can be used to understand how groundwater flows through the groundwater-bearing units in this narrow valley, which is constricted on both sides and below by basement rocks (figs. 5, 7). Through this valley, surface water (when present in Cahuilla Creek) and groundwater flow from the upper part of the Cahuilla Valley groundwater basin (northeast) to its lower part (southwest); the geometry of this constriction and the potential quantity of groundwater that flows through it have not been previously studied. The two monitoring sites that were installed in 2018 and data from the ERT profiles not only enable groundwater-level monitoring and an understanding of the geometry at those sites, but also make it possible to determine the groundwater-level gradient between them. There are differences in the groundwater elevations between the two monitoring sites and between the wells at each site. In April 2019, the groundwater elevations in the wells 7S/3E-29M2 and -29M3 at the upgradient profile 1 were about 28 ft higher compared to the shallow well 7S/3E-30Q3 at the downgradient profile 2—all of which were completed in the alluvium (figs. 5, 7, 8). In October 2019, the difference in groundwater elevations in the alluvium between the two ERT profiles, which are about 2,150 ft apart, was about 39 ft. The deep well at the downgradient profile was artesian when it was installed and was observed flowing in 2021, indicating an upward gradient at this site. Because artesian conditions indicate that the hydraulic head, or pressure head,

in the groundwater-bearing unit in which a well is perforated is above land surface elevation, the upward gradient at this site indicates that the basement and decomposed basement contribute groundwater to the overlying alluvium laterally, from below, or both. This quantity of groundwater likely is small but is enough to cause the hydraulic head in the basement unit to be about 12 ft higher in well 7S/3E-30Q2 than in well 7S/3E-30Q3 in the overlying alluvium (fig. 8). Using data obtained from the ERT profiles and the monitoring wells, it is possible to estimate the rate at which groundwater flows within the alluvium. As discussed in the “Electrical Resistivity Tomography” section, the cross-sectional area of the alluvium along the upgradient profile 1 (fig. 7) was about 67,000 ft2. Although this is a larger area than at profile 2, this profile can be used to estimate a high-end value for flow. The lithology encountered during the auguring was often difficult to distinguish at depth because of the method used, but most of the subsurface lithology was described as sand with silts and some clay (fig. 8). Assuming that the alluvium is homogeneous and that monitoring wells are parallel to the direction of groundwater flow, estimates of lateral groundwater flow can be made using Darcy’s Law: Q = Kai where

is flow, in ft/day;

is hydraulic conductivity, in ft/day;

is area, in ft2; and

is hydraulic gradient, in ft/ft.

(1)

Published values of hydraulic conductivity were used for silty and clean sands that range from 1 to 15 ft/day (Freeze and Cherry, 1979), and April 2019 groundwater levels in the alluvium were used (resulting in a hydraulic gradient of about 0.013 ft/ft); estimates of lateral groundwater flow within the alluvium between the two ERT profiles ranged from less than 10 acre-ft/yr to about 110 acre-ft/yr. The hydraulic gradient in October 2019 was slightly higher at about 0.018 ft/ft but resulted in similar estimates of flow, and estimates of groundwater flow within the alluvium at that time ranged from about 10 to 150 acre-ft/yr. The actual volume of groundwater flow between the two ERT profiles may be higher if coarser sediments with higher hydraulic conductivity values are present in parts of the narrow valley, or if lenses of coarser sediments are connected and are laterally extensive. Conversely, the volume may be lower if finer-grained sediments are present, which would lower the overall hydraulic conductivity and subsequent transmissivity of the groundwater-bearing deposits.

50 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Short-Term Trends in Groundwater Levels An examination of short-term trends in groundwater levels can show how some parts of the basin respond quickly to recharge and discharge, at least hydrologically, because depths to groundwater are shallow in many places and are affected by the variable cycles of wet or dry climatic periods. The effects of the interannual variability on recharge in this arid region were discussed in the “Natural Recharge” section and are evident when comparing groundwater-level responses after short-term precipitation events. To show any potential effects of short-term local recharge events and stresses from pumpage in different areas, hydrographs were constructed using groundwater-level elevations from August 2017 to December 2021 from 10 wells and combined with data collected from two precipitation sites installed as part of this study (figs. 14, 15). The short-term responses to precipitation events provide information about the thickness of the alluvium and depth of the decomposed and competent basement, and the effects of local recharge, if present. Groundwater levels in wells that are outside the Cahuilla Valley groundwater basin, or in areas where the alluvium is thin, showed more pronounced responses from precipitation events than wells where the alluvium is thickest—mostly within the groundwater basin boundary (fig. 9A). For example, the hydrograph for well 7S/2E-13R1 (hydrograph 1 on figs. 14 and 15) shows that groundwater levels increased rapidly after a large storm event on February 14, 2019. As a result of that storm and others in the following winter and spring months, groundwater levels in well 7S/2E-13R1 rose almost 50 ft by June 2019, demonstrating a response to local recharge over about a 4-month period. This well is not near any local creeks, and according to the driller’s log (California Department of Water Resources, 2022), it is completed mostly in decomposed basement and competent basement which extend to at least 70 ft bls in this well. Heavy local precipitation may have occurred near the well that was not recorded by the precipitation gages, but the response in this well more likely represents lateral movement of groundwater through the decomposed basement. The rapid response to storm events in this and other wells completed in the decomposed basement and competent basement shows that local recharge infiltrates and then dissipates quickly and that these materials do not have much capacity for the long-term storage of groundwater compared to the alluvium. Another well on the edge of the Cahuilla Valley groundwater basin boundary also showed a rapid, but more muted and prolonged response, to the storms in early 2019. Well 7S/3E-20A2 (figs. 14, 15, hydrograph 2) is about 100 ft from Cahuilla Creek, which had an estimated peak flow

of 59 ft3/s from the February 2019 storm event (table 1). Groundwater-level elevations in this well rapidly increased by almost 5 ft, reached a peak in early March, and plateaued through May (figs. 14, 15, hydrograph 2). In this well, the rise in groundwater elevations began about 3 weeks after the largest storm event in February 2019. The driller’s log from this well indicates that it is perforated in granite (California Department of Water Resources, 2022), or basement rocks, as described in this report. Wells that are further within the Cahuilla Valley groundwater basin, such as wells 7S/3E-21D4 and 7S/3E-23D3 (figs. 14, 15, hydrographs 3, 4), showed little response to the storms in the winter and spring of 2019. These wells are in the area where the alluvium is much thicker and have small but steady groundwater-level declines. For example, the groundwater levels in well 7S/3E-21D4 declined by slightly less than 1 ft between 2018 and 2021. Other wells, such as 7S/3E-34E1 (figs. 14, 15, hydrograph 6), show small, short-term fluctuations in groundwater levels, which are mostly due to varying pumping cycles in nearby wells. Well 7S/3E-13C2 (figs. 14, 15, hydrograph 5) had subtle changes in groundwater levels, but these changes were not concurrent with any recorded precipitation event(s) nor did they show any effect from nearby pumping. The groundwater levels in this well increased steadily over 4 ft from 2019 to 2020 and reached a maximum in summer 2020, then steadily decreased through December 2021. This well is within the San Jacinto fault zone and showed almost no changes in groundwater levels related to precipitation events. The data for all the wells discussed herein are available on the USGS NWIS website (U.S. Geological Survey, 2021), as described in the “Accessing Data” section. In general, the differences in groundwater-level responses in wells were due to several factors, including but not limited to, the following: (1) the hydrologic properties of the aquifer to transmit water both vertically and horizontally; (2) proximity to recharge sources; (3) variation in the spatial and temporal distribution of precipitation; (4) hydraulic properties of the aquifer to store water; (5) the thickness of the alluvium or basement materials in which wells are screened; or (6) a combination of these factors. Short-term groundwater-level fluctuations occurring over weeks or months are indicative of the limited capacity of the aquifer system for groundwater storage in some areas and the sensitivity of this basin to the amount and availability of recharge. In areas where sustained historical pumpage has occurred, primarily from the alluvium for agriculture near and northeast of the town of Anza, groundwater levels are deeper and generally do not show the short-term effects of local recharge from storm events or surface runoff.

Results 51 116°50'

116°45'

116°40'

116°35'

THOMAS MTN PRECIP GAGE NR ANZA CA 7S/3E-21D4

7S/3E-23D3

7S/3E-20A2

T. 6 S.

7S/3E-13C2

33° 35'

7S/2E-13R1 T. 7 S.

Anza

Durasno Valley 007S003E34E001S PRECIP 33° 30'

CIN

FA U

LT Z

8 7S/3E-34E1

7 7S/3E-30Q2 7S/3E-30Q3

371

T. 8 S. 7S/3E-29M2 7S/3E-29M3 RIVERSIDE COUNTY SAN DIEGO COUNTY

33° 25'

T. 9 S.

R. 1 E. R. 2 E. Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

R. 3 E. 4 MILES

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION Groundwater basin boundary (California Department of Water Resources, 2020)

Study area boundary Fault—Dashed where approximately located, dotted where concealed

Cahuilla Valley Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023)

THOMAS MTN PRECIP GAGE NR ANZA CA

Cahuilla Reservation Ramona Band of Cahuilla Reservation

7S/2E-13R1

Precipitation gage and identifier

Thumbnail of hydrograph shown in figure 15

Well and identifier

Figure 14. Location of wells with short-term hydrographs and precipitation data shown on figure 15 near Anza, California (U.S. Geological Survey, 2021).

3,850

3,840

3,830

3,820

3,810

3,800

3,890

Approximate land surface 3,939 feet above North American Vertical Datum of 1988 (NAVD 88)

3,880

3,870

3,860

3,850

3,840 2017

2018

2019

2020

2021

Daily precipitation, in inches

Groundwater-level elevation, in feet above NAVD 88

3,860

Approximate land surface 3,876 feet above North American Vertical Datum of 1988 (NAVD 88)

Groundwater-level elevation, in feet above NAVD 88

52 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

0 2022

Calendar year EXPLANATION Groundwater-level well 7S/3E-20A2 3,790 2017

2018

2019

2020

2021

Precipitation station

0 2022

007S003E34E001S PRECIP THOMAS MTN PRECIP GAGE NR ANZA CA

Calendar year EXPLANATION Groundwater-level well 7S/2E-13R1 Precipitation station 007S003E34E001S PRECIP

3,870

3,860

3,850

3,840

3,830 2017

2018

2019

2020

2021

Calendar year

0 2022

3,190

Approximate land surface 3,984 feet above North American Vertical Datum of 1988 (NAVD 88)

3,920

3,910

3,900

3,890

3,140 2017

2018

2019

2020

2021

Daily precipitation, in inches

Groundwater-level elevation, in feet above NAVD 88

3,880

Approximate land surface 3,935 feet above North American Vertical Datum of 1988 (NAVD 88)

Daily precipitation, in inches

Groundwater-level elevation, in feet above NAVD 88

THOMAS MTN PRECIP GAGE NR ANZA CA

0 2022

Calendar year

EXPLANATION

Groundwater-level well 7S/3E-21D4

Groundwater-level well 7S/3E-23D3

Precipitation station

007S003E34E001S PRECIP

THOMAS MTN PRECIP GAGE NR ANZA CA

Figure 15. Groundwater-level hydrographs (2017–21) from wells and precipitation sites near Anza, California (U.S. Geological Survey, 2021; location of wells and precipitation sites shown on fig. 14).

4,170

4,160

4,150

3,790

2019

2018

2020

2021

0 2022

3,820

3,810

3,800

3,790 2017

2018

2019

2020

2021

Calendar year EXPLANATION

Groundwater-level well 7S/3E-13C2

Groundwater-level well 7S/3E-34E1

Precipitation station

007S003E34E001S PRECIP

THOMAS MTN PRECIP GAGE NR ANZA CA

3,780

3,770

3,760

3,750

3,740 2017

Calendar year

Approximate land surface 3,773 feet above North American Vertical Datum of 1988 (NAVD 88)

2019

2018

2020

2021

Calendar year

0 2022

3,760

0 2022

Approximate land surface 3,733 feet above North American Vertical Datum of 1988 (NAVD 88)

3,750

3,740

3,730

3,720

3,710 2017

2018

2019

2020

2021

Calendar year

EXPLANATION

Groundwater-level well 7S/3E-29M2

Groundwater-level well 7S/3E-30Q2

Groundwater-level well 7S/3E-29M3

Groundwater-level well 7S/3E-30Q3

Precipitation station

007S003E34E001S PRECIP

THOMAS MTN PRECIP GAGE NR ANZA CA

Figure 15.—Continued

3,830

EXPLANATION

Groundwater-level elevation, in feet above NAVD 88

4,140 2017

Daily precipitation, in inches

4,180

3,840

Approximate land surface 3,879 feet above North American Vertical Datum of 1988 (NAVD 88)

0 2022

Daily precipitation, in inches

4,190

Groundwater-level elevation, in feet above NAVD 88

Approximate land surface 4,211 feet above North American Vertical Datum of 1988 (NAVD 88)

Daily precipitation, in inches

Groundwater-level elevation, in feet above NAVD 88

Results 53

54 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Long-Term Trends in Groundwater Levels Compiling long-term groundwater-level data from wells gives insight as to how stresses such as recharge and discharge (mainly pumpage) have affected the movement and direction of groundwater flow over decades. In most areas, a historical record of continuous groundwater-level data from a single well is rare, so data from wells that are near each other are often combined. Using groundwater-level data that were available from 1916 (Waring, 1919), 18 long-term hydrographs were constructed within the Cahuilla Valley groundwater basin, showing trends from 1916 to 2021. Most wells near the boundaries and outside of the Cahuilla Valley and Terwilliger Valley groundwater basins showed more rapid responses to stresses, and some wells outside of the groundwater basin had groundwater levels that remained mostly unchanged or had very little decline. In contrast, wells within the groundwater basins generally showed the effects of long-term pumping. The data collected between 1916 and 2021 show that changes in groundwater levels within the boundary of the Cahuilla Valley groundwater basin were variable and dependent on location. As mentioned earlier, wells outside of both groundwater basins showed little or no long-term groundwater-level declines in contrast to wells within the groundwater basins. Wells within the groundwater basins had variable trends in groundwater levels depending on their location relative to recharge that originates mostly in the higher elevations and areas of long-term groundwater withdrawal for agricultural pumpage. Groundwater levels in wells that are farthest from where most recharge occurs in the northeast had some of the largest long-term groundwater-level declines. Groundwater levels in areas farther from recharge sources do not respond rapidly to recharge events because recharge takes longer to arrive at these areas. For example, well 7S/3E-34E1 (figs. 16, 17, hydrograph 11), which is in the southeastern part of the Cahuilla Valley groundwater basin, declined by about 30 ft between 1951 and 2021, less than about 0.5 ft/year; groundwater levels in well 8S/3E-2D1, which is in the northwestern part of the Terwilliger Valley groundwater basin, declined by about 40 ft between 1960 and 2021, or about 0.7 ft/year (figs. 16, 17, hydrograph 10). In addition to the distance these wells are from the recharge sources in the higher elevations, the depth to groundwater ranged between

60 and 80 ft bls, so any potential locally-derived recharge would take longer to migrate downward to the water table than in places where the groundwater table is shallower (see the “Short-Term Trends in Groundwater Levels” section). Areas of long-term agricultural pumpage had the largest groundwater-level declines because pumping has controlled the movement and direction of groundwater flow. Combining the records from one of the wells Waring (1919) reported in 1916 with the more recent measurements from well 7S/3E-22J2 shows that groundwater-levels declined about 60 ft between 1916 and 2021 (figs. 16, 17, hydrograph 8). Groundwater levels in well 7S/3E-14P3 have declined almost 40 ft over the shorter period between 1971 and 2021—a rate of about 0.8 ft/year (figs. 16, 17, hydrograph 6). Although this well is in the northeastern part of the Cahuilla Valley groundwater basin and is near recharge sources, the depth to groundwater in this well dropped from about 75 ft bls to more than 112 ft bls over those 50 years. The long-term data show that several factors affect groundwater flow and levels. In addition to a well’s distance from recharge sources, some quantity of recharge is captured upgradient by pumping wells in the northeastern and southern parts of the groundwater basins, which diminishes the amount of recharge that reaches downgradient areas. The long-term declines in groundwater levels also suggest that the amount of water removed by pumpage has exceeded that which has been recharged and that groundwater has been removed from the aquifer system; that is, the amount of groundwater from aquifer storage has decreased. Moyle (1976) estimated that the total volume of groundwater removed from aquifer storage, or the amount of groundwater depletion, between 1950 and 1973 in two areas in the Cahuilla Valley groundwater basin was about 14,000 acre-ft, or about 600 acre-ft/yr. Long-term groundwater-level data are invaluable for understanding changes in groundwater flow, trends through time, and the effects of recharge and discharge. These long-term data indicate where imbalances between the volume of water that is recharged and amount of groundwater that is discharged, primarily by pumpage, exist. The continued evaluation of these long-term data is important to successfully managing the basins’ groundwater resources and exploring potential management strategies in the future.

Results 55 116°50'

116°45'

116°40'

7S/3E-9E1

116°35'

7S/3E-9D1

7S/3E-10B1

33° 35'

7S/3E-13C1

7S/3E-11P3

7S/3E-17H3

T. 6 S.

7S/2E-15A4

172

Anza

166

7S/3E-14P3

171

Durasno Valley

7S/3E-22J2

7S/3E-16N5

7S/3E-23D1

CIN

7S/2E-23K1 33° 30'

T. 7 S.

FA U

LT Z

8S/4E-7A1

7S/3E-21L1 371

8S/3E-2D1

T. 8 S.

11 12 7S/3E-22D3

7S/3E-15P1 7S/3E-34E1

RIVERSIDE COUNTY SAN DIEGO COUNTY

33° 25'

T. 9 S.

R. 1 E.

R. 2 E.

R. 3 E. 0

Base from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator, zone 11 north; North American Datum of 1983

2 2

4 MILES

R. 4 E. Faults modified from California Department of Water Resources (1974); Jennings and others (2010)

4 KILOMETERS

EXPLANATION Groundwater basin boundary (California Department of Water Resources, 2020)

Study area boundary

Cahuilla Valley

Well and identifier with hydrograph

Terwilliger Valley Tribal land boundary (U.S. Bureau of Indian Affairs, 2023) Cahuilla Reservation

Thumbnail of hydrograph shown in figure 17

Fault—Dashed where approximately located, dotted where concealed

170

8S/3E-2D1

Well and identifier, in feet above North American Vertical Datum of 1988 (Waring, 1919)

Ramona Band of Cahuilla Reservation

Figure 16. Location of wells with long-term hydrographs shown on figure 17 near Anza, California (U.S. Geological Survey, 2021).

Approximate land surface at well 4,231 feet above NAVD 88

4,130 4,120 4,110 4,100 4,090 4,080

4,050

4,210 4,200 4,190 4,180 4,170

EXPLANATION

4,160

Well 7S/3E–9D1

4,150 4,140

Calendar year

3,900

Calendar year

4,120

Well 7S/3E–14P3

3,910

4,130

EXPLANATION

3,920

Well 7S/3E–13C1

4,140

3,930

EXPLANATION

3,940

4,150

3,950

4,160

3,960

4,170

4,180

3,970

4,190

3,980

4,200

Approximate land surface at well 4,032 feet above NAVD 88

3,990

4,000

Approximate land surface at well 4,207 feet above NAVD 88

4,210

Calendar year

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

4,220

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

3,730

4,270

Well 7S/3E–11P3

3,740

4,280

EXPLANATION

3,750

Well 7S/3E–10B1

3,760

EXPLANATION

4,290

3,770

4,300

3,780

4,310

3,790

4,320

3,800

4,330

3,810

4,340

Approximate land surface at well 4,120 feet above NAVD 88

3,820

4,350

4,360

3,830

Approximate land surface at well 4,360 feet above NAVD 88

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

4,370

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

Well 7S/3E–9E1

4,220

4,060

4,230

EXPLANATION

Approximate land surface at well 4,316 feet above NAVD 88

4,070

4,240

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

4,140

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

4,150

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

56 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Calendar year

Figure 17. Groundwater-level hydrographs (1915–2021) from wells near Anza, California (U.S. Geological Survey, 2021). Location of wells shown on figure 16.

Approximate land surface at well 3,978 feet above NAVD 88

3,960 3,950 3,940 3,930 3,920 3,910

EXPLANATION Well 7S/3E–23D1

3,880

3,970

3,960 3,950 3,940 3,930 3,920

EXPLANATION

3,910

Well 7S/3E–22J2

3,900

Well 172 (Waring, 1919)

3,890 3,880

Calendar year

Figure 17.—Continued

3,880

Calendar year

15 20

3,870

Calendar year

3,770

3,890

3,780

3,900

Well 7S/3E–34E1

3,910

EXPLANATION

3,790

3,920

3,800

Well 172 (Waring, 1919)

3,930

3,810

3,940

3,820

EXPLANATION Well 7S/3E–15P1

3,830

3,950

3,840

3,960

3,850

Approximate land surface at well 3,939 feet above NAVD 88

3,970

Approximate land surface at well 3,879 feet above NAVD 88

3,860

Calendar year

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,870

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

3,760

3,750

Well 8S/3E–2D1

3,770

3,760

EXPLANATION

3,780

Well 8S/4E–7A1

3,770

3,790

EXPLANATION

3,800

3,780

3,810

3,790

3,820

3,800

3,810

3,830

3,820

3,840

3,830

Approximate land surface at well 3,882 feet above NAVD 88

3,850

3,840

3,860

Approximate land surface at well 3,862 feet above NAVD 88

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,850

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

3,890

Approximate land surface at well 3,931 feet above NAVD 88

3,900

3,980

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

3,970

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,980

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Results 57

Approximate land surface at well 3,905 feet above NAVD 88

3,930 3,920 3,910 3,900 3,890 3,880

3,850

3,830 3,820 3,810 3,800

EXPLANATION

3,790

Well 7S/3E–21L1

3,780

Well 171 (Waring, 1919)

3,770 3,760

Calendar year

Figure 17.—Continued

Well 7S/3E–17H3

3,880

Well 166 (Waring, 1919)

Calendar year

3,870

Calendar year

3,750

EXPLANATION

3,890

Well 7S/2E–15A4

3,760

3,900

EXPLANATION

3,770

3,910

3,780

3,920

3,790

3,930

3,800

3,810

3,940

3,820

3,950

3,830

Approximate land surface at well 4,031 feet above NAVD 88

3,960

3,970

Approximate land surface at well 3,812 feet above NAVD 88

3,840

Calendar year

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,850

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

3,570

3,800

Well 7S/2E–23K1

3,580

3,810

EXPLANATION

3,590

Well 7S/3E–16N5

3,820

3,600

EXPLANATION

3,610

3,830

3,620

3,840

3,630

3,850

3,860

3,640

3,870

3,650

3,880

Approximate land surface at well 3,653 feet above NAVD 88

3,660

3,890

3,670

Approximate land surface at well 3,943 feet above NAVD 88

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,900

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

Calendar year

Well 7S/3E–22D3

3,840

3,860

3,850

EXPLANATION

Approximate land surface at well 3,851 feet above NAVD 88

3,870

3,860

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

3,940

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

3,950

19 15 19 25 19 35 19 45 19 55 19 65 19 75 19 85 19 95 20 05 20 15 20 25

Groundwater-level altitude, in feet above North American Vertical Datum of 1988 (NAVD 88)

58 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

Summary 59

Summary Groundwater is the sole source of water supply for a rural community and two Native American Tribes in the Anza Valley, California, and the relation between the groundwater-bearing units of the groundwater system and the amount of natural recharge to the Cahuilla Valley and Terwilliger Valley groundwater basins is not well understood. During the 20th century, the reliance on groundwater for agricultural, domestic, and municipal uses often has exceeded recharge, and there is the potential for changes in groundwater availability related to climate change. To better manage the water resources in the area, the Ramona Band of Cahuilla and the U.S. Geological Survey (USGS) initiated a cooperative study to understand the hydrogeologic system encompassing the Cahuilla Valley and Terwilliger Valley groundwater basins and surrounding area. Increasing groundwater use has raised concerns about potential changes in water sustainability. The purpose of this long-term, multi-phase study is to characterize the hydrogeology of the Cahuilla Valley and Terwilliger Valley groundwater basins and surrounding groundwater-bearing units, with the ultimate goal of developing a calibrated integrated hydrologic model to manage the groundwater supplies on a sustainable basis in the future. The purpose of this report is to describe the following: field data methods and interpretations, the development of the geologic framework model, the conceptual understanding of the hydrogeologic system, and the hydrologic stresses and changes in groundwater levels and flow through time. Wells inventoried in the early 1900s were used for homesteads and to grow grain; in the late 1940s, crops were transitioning to alfalfa. By the 1950s, groundwater levels had declined about 7 feet relative to 1916. Publications by the California Department of Water Resources from 1956 and 1974 documented declines in groundwater levels since the 1950s, initiating more focused studies aimed at aquifer characterization to better quantify the effects of groundwater depletion. Most of the groundwater pumped from the area has been from the alluvium in the Cahuilla Valley and Terwilliger Valley groundwater basins; the underlying competent and decomposed basement rocks also yield groundwater and mainly serve as domestic supply. The abundance of wells completed with perforations that span the alluvium, the decomposed basement, and competent basement indicate that both units are important for water supply and likely are interconnected. Groundwater, precipitation, and surface-water data were collected during this study to augment existing data to help better understand the hydrogeology of the groundwater basins. Since summer 2017, the USGS has collected discrete and continuous groundwater-level data from 90 wells throughout the study area as part of a long-term monitoring network. Pressure transducers were installed in 18 of the wells in 2021

to measure groundwater levels at 1-hour intervals to capture rapid changes in response to local stresses. Two precipitation sites were installed so that the amount of precipitation at higher elevations could be compared to what is received at the lower elevation in the Anza Valley. Two electrical resistivity tomography (also known as direct-current resistivity) surveys were done about 2,150 feet apart to identify the thickness of the alluvium, its horizontal extent, and the depth-to-basement along two profiles perpendicular to Cahuilla Creek across a narrow section of Durasno Valley. The interpretation of the two resistivity profiles showed increasing resistivity with depth and mostly horizontally layered sediments. The resistivity of the alluvium was generally between 10 and 75 ohm-meters. The transitional boundary between the alluvium and basement, which likely represents a zone of decomposed basement, was thinner and shallower along the upgradient profile where Cahuilla Creek enters the Durasno Valley; there, the depth-to-basement was shallowest at about 70 feet below land surface. The depth-to-basement was greater at the downgradient profile; the basement was greater than 135 feet below land surface. The results from the electrical resistivity tomography profiles were used to place four monitoring wells at two sites in the Durasno Valley in 2018 using auger drilling methods. As part of the drilling and construction of the four monitoring wells, downhole geophysical logs were collected to evaluate the subsurface lithology and to verify the electrical resistivity tomography results. Groundwater-level data from the wells in April 2019 showed that the hydraulic heads in both upgradient wells were about 6 feet below land surface but differed at the downgradient wells. At the downgradient site, the hydraulic head in the shallower well was about 12 feet lower than the deeper well, which was artesian; this finding indicates that the decomposed and competent basement contribute groundwater to the overlying alluvium in this valley. A digital three-dimensional geologic framework model was constructed to represent the subsurface geometry of the alluvium, decomposed basement, and competent basement within the study area. Lithology information from drillers’ log descriptions from about 1,185 well logs provided information on the subsurface geometry of the alluvium, decomposed basement, competent basement, and the variability of major lithologic textures within the alluvium. Interpreted surface and subsurface contacts for the geologic units were used as input data to construct the geologic framework model using EarthVision (version 11.0) software. Maps were made of the modeled thicknesses of the alluvium and decomposed basement, and the modeled elevation of the top of competent basement. Sections from the geologic framework model show the geometric relation of the modeled units, faults, and depth of selected wells. Additionally, the sections show the interpolated lithologic texture distribution within the alluvium.

60 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins The sources of natural recharge within the study area are mostly from the surrounding higher elevation areas of the San Jacinto Mountains to the northeast and from southeast of the Terwilliger Valley, but also include precipitation and runoff that mostly originate in the surrounding mountains and hills. Hamilton and Cahuilla Creeks and the many small unnamed creeks in the study area are ephemeral; no streams from adjacent surface-water basins drain into the study area. Recharge and runoff do not occur in large amounts every year, and when very wet years occur, most of the water becomes runoff, and a lesser component becomes recharge; both are controlled by the variable cycles of wet and dry weather and are temporally variable across the region. In addition to being temporally variable, recharge and runoff are spatially variable; recharge frequently occurs outside of the groundwater basins and rarely over the groundwater basins’ footprint. As part of this study, the regional-scale Basin Characterization Model for California was calibrated to estimate the potential in-place recharge and potential runoff into the groundwater system. Recharge and runoff have extreme interannual variability in arid regions, including the study area; the occurrence of recharge and runoff can be sporadic with many years lacking any substantive amount, so the long-term average value for recharge is not a reliable estimate for a particular year. Estimates of average annual recharge from a previous study and the Basin Characterization Model for California for four different periods ranged from 3,800 acre-feet per year (acre-ft/yr) for 1897–1947 to 5,900 acre-ft/yr for 1971–2000. Estimates of potential recharge from the disposal of domestic septic systems in this rural area were made from population and water-use data. Based on those data, the amount of potential recharge from septic effluent may have been as much as 500 acre-ft in 2020. The natural sources of groundwater discharge are the evapotranspiration by vegetation, evaporation from open water bodies, areas where groundwater is at or near land surface, and groundwater underflow that exits the groundwater basins to the west and southeast. Estimates of potential evapotranspiration rates for selected years were made by applying crop coefficient values to publicly available land-use data. Some evaporation occurs from open water bodies and in areas where groundwater is near land surface. The combined potential amount of water evaporated from the open water bodies was estimated to be about 400 acre-ft/yr. Evaporation estimates from about 485 acres along Cahuilla Creek were about 2,400 acre-ft/yr for wet years; this quantity is likely much less in drier years. Groundwater has been used as the sole source of water for Native American Tribes, municipal, and domestic supply, but most of the groundwater has been used for agriculture, which likely started near the turn of the 20th century. Estimates for agricultural and municipal pumpage for

1991–2021 were compiled for this report available from the Santa Margarita River Watershed Watermaster; these estimates were for users who irrigate 8 or more acres or use the equivalent quantity of water. Domestic groundwater use was estimated from population and per capita water use within the study area boundary. The estimated total pumpage for 1991–2021 ranged from about 1,140 acre-ft in 2019 to about 3,450 acre-ft in 1994. When summed, the cumulative amount of estimated pumpage between 1991 and 2021 was about 81,400 acre-ft. The direction of groundwater flow is generally from along the San Jacinto fault zone in the northeast at the headwaters of Cahuilla and Hamilton Creeks, to the surface-water outlets at the western and southeastern parts of the study area. From the higher elevations, groundwater generally flows westward through the Cahuilla Valley sloping with Cahuilla and Hamilton Creeks; the depth to groundwater is shallower southwest of the town of Anza and is deepest adjacent to the fault zone in the Cahuilla Valley groundwater basin. Maps depicting the groundwater elevations and flow directions through time were compiled for the years 1950, 1973, 1986, and 2021. The groundwater-level contours from 1950 show pumping depressions, indicating that some areas had been affected by pumpage. The data from 1950 also indicate that there was a natural groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins. The contours of the 2021 data show that pumping continues to affect groundwater levels in the Cahuilla Valley groundwater basin between the town of Anza and the northern boundary of the Cahuilla Reservation. The variability and extent of the groundwater elevations in the area over time indicate that the location of the natural groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins has migrated over time. Data from wells that are closer to the boundary of these two groundwater basins would help clarify groundwater flow and the location of the groundwater boundary under current conditions. Groundwater data from structurally complex areas in the Cahuilla Valley groundwater basin point to mechanisms other than recharge and discharge that affect groundwater flow, such as the San Jacinto fault zone. On a smaller scale, the data collected in the Durasno Valley were used to estimate the amount of groundwater that flows through the groundwater-bearing units in this narrow valley, which is constricted on both sides and underneath by basement rocks. Using Darcy’s Law, the geometry of the valley from the two electrical resistivity tomography profiles, groundwater levels from the monitoring wells installed near those profiles, and assumptions about the hydraulic properties of the alluvium, it was estimated that groundwater flow through the alluvium in the Durasno Valley in 2021 ranged from about 10 to 150 acre-ft/yr.

Summary 61 An examination of short-term trends in groundwater levels demonstrates how some parts of the basin react quickly, at least hydrologically, to local recharge and discharge stresses and the variable cycles of wet or dry climatic periods. Groundwater levels in wells that are outside the Cahuilla Valley and Terwilliger Valley groundwater basins, or in areas where the alluvium is thin, showed more pronounced responses from precipitation events than wells that are completed in the alluvium within the groundwater basin boundary. The rapid response to wells completed in the decomposed basement and competent basement likely is due to less pore space available for groundwater storage. Wells farther within the Cahuilla Valley groundwater basin perforated in the alluvium showed much less of a response to the storms in the winter and spring of 2019. Most of the recharge originates from the higher elevations and takes longer to arrive at wells in the valley, and the short-term fluctuations in groundwater levels in these wells mostly are due to pumpage cycles during the growing season. In general, the differences in how groundwater levels respond in wells may be due to several factors, including, but not limited to, the following: (1) the hydrologic properties of the aquifer to transmit water both vertically and horizontally; (2) proximity to recharge sources; (3) variation in the spatial and temporal distribution of precipitation; (4) hydraulic properties of the aquifer to store water; (5) the thickness of the alluvium or basement materials in which wells are completed; or (6) a combination of these factors. In areas where sustained historical pumpage has occurred in the alluvium, primarily for agriculture near and northeast of the town of Anza,

groundwater levels have declined and generally do not show the short-term effects of local recharge from storm events or runoff. The groundwater-level data collected between 1916 and 2021 show that changes in groundwater levels across the Cahuilla Valley groundwater basin are variable and dependent on location. Wells outside of the groundwater basins showed little or no long-term groundwater-level declines in contrast to wells within the groundwater basins. Groundwater levels in some wells that are farthest from where most of the recharge occurs had some of the largest long-term groundwater-level declines. Groundwater levels in wells that are farthest from areas where most recharge occurs do not respond rapidly to recharge events because recharge takes longer to arrive at these areas. Areas of long-term agricultural pumpage had the largest groundwater-level declines, and the pumping has affected the movement and direction of groundwater flow. Groundwater levels in some wells have declined by about 40 feet over the period of 1971–2021, a rate of about 0.8 foot/year. Near areas of recharge, the depths to groundwater in some wells have dropped during that period because of long-term pumpage. The long-term groundwater-level data show that several factors affect groundwater flow and levels. In addition to a well’s distance from recharge sources, some quantity of recharge is captured upgradient by pumping wells, which diminishes the amount of recharge that reaches downgradient areas. The continued evaluation of these long-term data is an important part of successfully managing the basins’ groundwater resources and exploring potential management strategies in the future.

62 Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins

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For more information concerning the research in this report, contact the Director, California Water Science Center U.S. Geological Survey 6000 J Street, Placer Hall Sacramento, California 95819 https://www.usgs.gov/centers/california-water-science-center Publishing support provided by the USGS Science Publishing Network, Sacramento Publishing Service Center

Stamos and others—Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley GW Basins—SIR 2025–5073

ISSN 2328-0328 (online) https://doi.org/10.3133/sir20255073

Hydrogeologic Characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California

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