Method to Identify Wells That Yield Water That Will Be Replaced by Colorado River Water in Arizona, California, Nevada, and Utah By RICHARD P. WILSON and SANDRAJ. OWEN-JOYCE
U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 94 4005
Prepared in cooperation with the BUREAU OF RECLAMATION
Tucson, Arizona 1994 replaced by water from the river and wells that The United States shall prepare and yield water that will be replaced by water from maintain, or provide for the preparation and maintenance of, and shall make available, precipitation or inflow from adjacent tributary annually and at such shorter intervals as the valleys. To aid in implementing the Supreme Secretary of the Interior shall deem necessary Court decree, a method was developed by the or advisable, for inspection by interested U.S. Geological Survey, in cooperation with the persons at all reasonable times and at a reasonable place or places, complete, detailed Bureau of Reclamation, to identify wells outside and accurate records of: * * * the flood plain of the lower Colorado River that yield water that will be replaced by water from * * * (B) Diversions of water from the main the river. The method provides a uniform stream, return flow of such water to the stream as is available for consumptive use in the criterion of identification that is based on United States or in satisfaction of the Mexican hydrologic principles for all users pumping treaty obligation, and consumptive use of such water from wells. water. These quantities shall be stated separately as to each diverier from the mainstream, each point of diversion, and each of the States of Arizona, California, and Legal Framework Nevada; * * *
The Colorado River Compact of 1922 Article I of the decree defines terminology apportions the waters of the Colorado River and states in part: between the upper basin States and the lower (A) "Consumptive use" means diversions basin States (U.S. Congress, 1948, p. A17-A22). from the stream less such return flow thereto as The requirement for participation of the U.S. is available for consumptive use in the United Geological Survey and the Bureau of States or in satisfaction of the Mexican treaty Reclamation is stated in Article V: obligation; (B) "Mainstream" means the mainstream The chief official of each signatory State charged with the administration of water rights, of the Colorado River downstream from Lee together with the Director of the United States Ferry within the United States, including the reservoirs thereon; Reclamation Service and the Director of the United States Geological Survey shall cooperate, ex-officio: (C) Consumptive use from the mainstream within a state shall include all consumptive uses of water of the mainstream, including (a) To promote the systematic determination water drawn from the mainstream by and coordination of the facts as to flow, underground pumping, and including but not appropriation, consumption, and use of water in limited to, consumptive uses made by persons, the Colorado River Basin, and the interchange by agencies of that state, and by the United of available information in such matters. States for the benefit of Indian reservations and other federal establishments within the Water in the lower Colorado River is state; * * * apportioned among the States of California, Arizona, and Nevada by the Boulder Canyon Project Act of December 21, 1928 (U.S. Purpose and Scope Congress, 1948, p. A213-A225) and confirmed by the U.S. Supreme Court decree, 1964, This report documents the method to identify Arizona v, California, in terms of consumptive wells outside the flood plain of the lower use. The decree is specific about the Colorado River that yield water that will be responsibility of the Secretary of the Interior to replaced by water from the river. The report account for consumptive use of water from the defines and delineates the river aquifer in the mainstream; consumptive use is defined to lower Colorado River valley in Arizona, include "water drawn from the mainstream by California, Nevada, and Utah (fig. 1); describes underground pumping." Article V of the decree the source of water in the river aquifer, and (U.S. Supreme Court, 1964) states in part: describes the sediments and sedimentary rocks
2 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY Robert M. Hirsch, Acting Director
For additional information Copies of this report can be write to: purchased from:
U.S. Geological Survey District Chief Earth Science Information Center U.S. Geological Survey Open-File Reports Section Water Resources Division Box 25286, MS 517 375 South Euclid Avenue Denver Federal Center Tucson, AZ 85719-6644 Denver, CO 80225 CONTENTS
Page
Abstract...... 1 Introduction...... 1 Legal framework...... 2 Purpose and scope...... 2 Data collection...... 4 Previous investigations...... 4 Acknowledgments...... 4 Concept of the river aquifer and accounting surface...... 6 Description of the river aquifer...... 8 Source of water in the river aquifer...... 8 Geologic formations and their hydrologic characteristics...... 9 Delineation of the river-aquifer boundary...... 10 Above Davis Dam...... 11 Davis Dam to Topock...... 12 Topock to Parker Dam...... 13 Parker Dam to Draper Lake...... 14 Clear Lake to Laguna Dam...... 15 Gravity studies by D. R. Pool...... 15 La Posa Plain...... 17 Vidal Valley...... 19 Chuckwalla Valley...... 21 Smoketree Valley...... 21 Generation of the accounting surface...... 26 Lakes Mead, Mohave, and Havasu...... 26 Davis Dam to Topock...... 27 Parker Dam to Draper Lake...... 27 dear Lake to Laguna Dam...... 27 Potential adjustments to the method...... 28 Summary...... 28 Selected references...... 29
PLATES
[Plates are in box] 1-19. Maps showing accounting surface and river aquifer in the lower Colorado River valley: 1. Clover Mountains, Nevada-Utah 2. Saint George, Utah-Arizona 3. Overton, Nevada-Arizona 4. Littlefield, Arizona 5. Lake Mead, Arizona-Nevada 6. Mount Trumbull, Arizona 7. Boulder City, Arizona-Nevada 8. Peach Springs, Arizona
Contents III CONTENTS Continued
PLATES Continued 9. Davis Dam, Arizona-Nevada-California 10. Needles, Arizona-California 11. Sheep Hole Mountains, California 12. Parker, Arizona-California 13. Alamo Lake, Arizona 14. Eagle Mountains, California 15. Blythe, Arizona-California 16. Salome, Arizona 17. Salton Sea, California 18. Trigo Mountains, Arizona-California 19. Yuma, Arizona-California
FIGURES 1. Map showing the lower Colorado River and area! extent of the river aquifer...... 3 2. Map showing index to maps of the river aquifer and accounting surface in the lower Colorado River valley, plates 1-19...... 5 3. Schematic diagrams showing the river aquifer and accounting surface...... 7 4. Map showing the complete-Bouguer gravity anomaly for La Posa Plain, Arizona...... 18 5. Profile and graph showing observed and simulated two-dimensional gravity model for La Posa Plain, Arizona...... 19 6. Map showing complete-Bouguer gravity anomaly for Vidal, California...... 20 7. Map showing isostatic-residual gravity anomaly for Chuckwalla Valley, California...... 22 8. Profile and graph showing observed and simulated two and one-half-dimensional gravity model for Chuckwalla Valley, California...... 23 9. Map showing complete-Bouguer gravity anomaly for Smoketree Valley, California...... 24 10. Profile and graph showing observed and simulated two-dimensional gravity model for Smoketree Valley, California...... 25
IV Contents CONVERSION FACTORS
Multiply By To obtain
inch 2.54 centimeter foot 0.3048 meter mile 1.609 kilometer square mile 2.590 square kilometer acre 0.4047 square hectometer acre-foot 0.001233 cubic hectometer cubic foot per second 0.02832 cubic meter per second gallon per minute 0.06309 liter per second
DEFINITION OF TERMS Selected hydrologic and geologic terms used in the report are defined below. Terms were adapted from Bates and Jackson (1987), Lohman and others (1972), Meinzer (1923), and U.S. Water-Resources Council (1980) or were defined in this report. Accounting surface (this report) The accounting surface represents the elevation and slope of the unconfmed static water table in the river aquifer outside the flood plain and the reservoirs of the Colorado River that would exist if the river were the only source of water to the river aquifer. The accounting surface was generated by using profiles of the Colorado River and water-surface elevations of reservoirs, lakes, marshes, and drainage ditches. Acre-foot The volume of water required to cover 1 acre to a depth of 1 foot; 43,560 cubic feet or 325,851 gallons. Aquifer A geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs. Bedrock (this report) Consolidated rocks that form the bottom and sides of the basins that underlie the Colorado River valley and adjacent tributary valleys and the mountain masses that rim the basins and valleys. The bedrock is nearly impermeable and is a barrier to ground-water flow. Flood plain A surface or strip of relatively smooth land adjacent to a river channel, constructed by the present river in its existing regimen and covered with water when the river overflows its banks. In this report, the flood plain is that part of the Colorado River valley that has been covered by floods of the modern Colorado River as it meandered prior to the construction of Hoover Dam. The flood plain commonly is bounded by terraces and alluvial slopes that rise to the foot of mountains that rim the valley. The flood plain ranges in width from less than 1,000 feet where the river is confined in bedrock canyons to about 9 miles in Palo Verde Valley. Geologic formation A persistent body of igneous, sedimentary, or metamorphic rock, having easily recognizable boundaries that can be traced in the field without recourse to detailed paleontologic or petrologic analysis, and large enough to be represented on a map as a practical or convenient unit for mapping and description. Formations are described in geologic literature and have formal names (Bouse Formation) or informal names (younger alluvium). River (this report) The Colorado River and associated reservoirs, lakes, marshes, and drainage ditches, unless otherwise specified. River aquifer (this report) The aquifer that consists of permeable, partly saturated sediments and sedimentary rocks that are hydraulically connected to the Colorado River so that water can move between
Contents V the river and the aquifer in response to withdrawal of water from the aquifer or differences in water-level elevations between the river and the aquifer. Sea level In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 A geodetic datum derived from a general adjustment of the first order level net of the United States and Canada, formerly called "Sea Level Datum of 1929." Section, Township (T.), and Range (R.) Locations used by the U.S. Geological Survey are in accordance with the Bureau of Land Management's system of land subdivision. The land survey in Arizona is based on the Gila and Salt River (G&SR) meridian and base line, in California is based on the San Bernardino (SB) meridian and base line, in Nevada is based on the Mount Diablo (MD) meridian and base line, and in Utah is based on the Salt Lake (SL) meridian and base line. Sediment Solid fragmental material that originates from weathering of rocks and is transported or deposited by air, water, or ice, or that accumulates by natural means such as chemical precipitation or secretion by organisms, and that forms in layers on the Earth's surface in unconsolidated form. Sediments generally consist of alluvium, mud, clay, silt, sand, gravel, boulders, carbonate muds, shell fragments, and organic material; in basins of interior drainage, sediments include salt (halite), gypsum, and other evaporite minerals. Sedimentary rocks Rocks resulting from consolidation of sediments. The rocks can be formed in marine, estuarine, and continental environments. Static water level The level of water in a well that is not being affected by ground-water withdrawal. The level to which water will rise in a tightly cased well under its full pressure head. Water table The surface in an unconfined aquifer at which pressure is atmospheric and below which the permeable material is saturated with water. The water table is the level at which water stands in wells that penetrate the uppermost part of an unconfined aquifer.
VI Contents Method to Identify Wells That Yield Water That Will Be Replaced by Colorado River Water in Arizona, California, Nevada, and Utah
By Richard P. Wilson and Sandra J. Owen-Joyce
Abstract
Accounting for the use of Colorado River water is required by the U.S. Supreme Court decree, 1964, Arizona v. California. Water pumped from wells on the flood plain and from certain wells on alluvial slopes outside the flood plain is presumed to be river water and is accounted for as Colorado River water. A method was developed to identify wells outside the flood plain of the lower Colorado River that yield water that will be replaced by water from the river. The method provides a uniform criterion of identification for all users pumping water from wells by determining if the elevation of the static water table at a well is above or below the accounting surface. Wells that have a static water-level elevation equal to or below the accounting surface are presumed to yield water that will be replaced by water from the river. Wells that have a static water-level elevation above the accounting surface are presumed to yield water that will be replaced by water from precipitation and inflow from tributary valleys. The method is based on the concept of a river aquifer and an accounting surface within the river aquifer. The river aquifer consists of permeable, partly saturated sediments and sedimentary rocks that are hydraulically connected to the Colorado River so that water can move between the river and the aquifer in response to withdrawal of water from the aquifer or differences in water-level elevations between the river and the aquifer. The subsurface limit of the river aquifer is the nearly impermeable bedrock of the bottom and sides of the basins that underlie the Colorado River valley and adjacent tributary valleys. The accounting surface represents the elevation and slope of the unconfined static water table in the river aquifer outside the flood plain and the reservoirs of the Colorado River that would exist if the river were the only source of water to the river aquifer. The accounting surface extends outward from the edges of the flood plain or a reservoir to the subsurface boundary of the river aquifer. Maps at a scale of 1:100,000 show the extent and elevation of the accounting surface from the area surrounding Lake Mead to Laguna Dam near Yuma, Arizona.
INTRODUCTION of Colorado River water is required by a decree (U.S. Supreme Court, 1964); a report that contains records of diversions, returns, and Flow in the Colorado River is regulated by a consumptive use of water by individual water series of dams, and releases of water through users is published annually (Bureau of these regulatory structures are controlled by Reclamation, 1965-92). the United States. Water stored in reservoirs is released to meet downstream water Water pumped from wells on the flood plain requirements, to make storage available for and from certain wells on alluvial slopes outside flood control, and to generate power. Water the flood plain is presumed to be river water and from the Colorado River is diverted or pumped is accounted for as Colorado River water. Water and used to irrigate croplands and to support pumped from some wells outside the flood plain wildlife habitat in the marshes along the river. has not been included in the accounting because Water also is pumped from wells in the Colorado the subsurface limits of the aquifer that is River valley and adjacent tributary valleys for hydraulically connected to the river were not agricultural, municipal, industrial, and domestic defined. No method was available for uses. In the United States, accounting for the use identifying wells that yield water that will be
Introduction 113°
EXPLANATION SEDIMENTS AND SEDIMENTARY ROCKS
BEDROCK
RIVER AQUIFER
STREAMFLOW- GAGING STATION
0 25 MILES I i i i i I I ''' 'I 0 25 KILOMETERS
32°30
Figure 1. The lower Colorado River and areal extent of the river aquifer.
Introduction 3 that transmit and store the water. The report also constitute the river aquifer of this report, describes the concept and development of an discussed the geologic structures and framework accounting surface and contains an index map of the lower Colorado River valley, and (fig. 2) and 19 maps (pis. 1-19) that show the described the occurrence and movement of approximate boundaries of the river aquifer, the ground water (Metzger, 1965, 1968; Olmsted, generalized surface extent of the sediments and 1972; Metzger and Loeltz, 1973; Metzger and sedimentary rocks that form the river aquifer, others, 1973; Olmsted and others, 1973). Major and the elevation and contours of the accounting emphasis of these studies was the ground-water surface. Because the study was regional in flow system beneath the flood plain and its scope, detailed site-specific investigations that relation to the Colorado River because few wells would be required to precisely define the extent, were available outside the flood plain to provide thickness, or hydraulic properties of the river water levels or samples for chemical analysis. aquifer were not included. Eberly and Stanley (1978) described the stratigraphy and origin of the late Tertiary rock units that were deposited in the present basins of Data Collection the study area and form an important part of the river aquifer. Several geohydrologic studies of The U.S. Geological Survey collected Lake Mead National Recreation Area provided hydrologic data for the study from 1990 to 1993. much of the geologic and hydrologic The study area includes the lower Colorado information needed to delineate the river aquifer River drainage area and parts of some adjacent above Davis Dam in the Lake Mohave and Lake valleys in Arizona, California, Nevada, and Utah Mead areas (Bentley, 1979a, b, c; Laney, 1979a, and extends from the east end of Lake Mead b, c, 1981; Bales and Laney, 1992; J.T. Bales and south to Laguna Dam (fig. 1). Most field work R.L. Laney, written commun., 1993). Regional was done on the alluvial slopes around Lakes geologic setting and Tertiary tectonic evolution Mead, Mohave, and Havasu and adjacent to the of the Lake Mead area were described by flood plain in Mohave, Chemehuevi, Parker, Bohannon (1984). The Virgin River depression Palo Verde, and Cibola Valleys. Additional was defined and delineated from geologic and work was done in Cactus and La Posa Plains, gravity data (Blank and Kucks, 1989) and Chuckwalla and Smoketree Valleys, and the seismic and gravity studies (Bohannon and Yuma Proving Ground. (See pis. 1-19.) others, 1993). Estimates of surface and Wells outside the flood plain were subsurface tributary inflow below Hoover Dam inventoried and water-level measurements were were compiled by Owen-Joyce (1987). made where owners permitted access to the wells Chemical and isotope analyses of ground water and measuring instruments could be inserted into and geochemical models were used to determine the well. Water-surface elevations in drainage the processes that control ground-water ditches and marshes and land-surface elevations chemistry and determine the origin and chemical at many wells and gravity data-collection points evolution of the ground water along the lower were determined by use of Global Positioning Colorado River (Robertson, 1991). In compiling System satellite surveys (Remondi, 1985). Well and generalizing the extent of the sediments and data are stored in a data base of the Arizona sedimentary rocks that form the river aquifer, 70 District of the U.S. Geological Survey, Tucson, geologic studies and maps were used in addition Arizona. to those cited above.
Previous Investigations Acknowledgments
Previous geohydrologic studies of the lower Jeffery C. Addiego, Robert N. Neff, Earl E. Colorado River valley from Davis Dam to Yuma Burnett, and Louis A. Miranda of the Bureau of defined and described the formations that Reclamation performed cooperative work and
4 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water 116° 30' 115 30' 114° 113°
EXPLANATION 7 PLATE NUMBER
SHEEP HOLE MOUNTAINS
25 MILES I i i i i I 0 25 KILOMETERS
33 LagunaDamJ lmPerialDa 32°30' Figure 2. Index to maps of the river aquifer and accounting surface in the lower Colorado River valley, plates 1-19. Introduction 5 provided equipment and aerial photographs for to identify those wells presumed to yield water field operations. E.F. Di Sanza and Jayne M. that will be replaced by water from the river by Harkins, Bureau of Reclamation, provided determining if the elevation of the static water Colorado River profiles below Davis Dam. The table at a well is above or below the accounting Colorado River Indian Tribes and the Fort surface. Mohave Indian Tribe kindly provided Wells that have a static water-level elevation hydrologic data and access to their reservations. equal to or below the accounting surface are Irrigation districts, municipal utilities, and water presumed to yield water that will be replaced by companies provided well records and allowed water from the river (fig. 3, wells labeled R). access to their wells. San Bernardino and Pumping water from a well completed in the Riverside Counties in California provided well river aquifer where the elevation of the static logs. Palo Verde Irrigation District provided water level in the well is below the elevation of water-level data for drainage ditches. Special appreciation is extended to the well drillers and the accounting surface will eventually cause the slope of the hydraulic gradient between the river landowners of the area who provided well and the well to be downward toward the well. records and access to their property. This, in turn, will result in the movement of water from the river into the river aquifer. CONCEPT OF THE RIVER AQUIFER Wells that have a static water-level elevation above the accounting surface are presumed to AND ACCOUNTING SURFACE yield water that will be replaced by water from precipitation and inflow from tributary valleys The method required the definition and (fig. 3B, wells labeled T). Water from tributary delineation of the river aquifer and the definition inflow that recharges the river aquifer creates and generation of the accounting surface within local ground-water mounds or ridges in the river the river aquifer. The river aquifer consists of aquifer that have static water-level elevations permeable, partly saturated sediments and above those of nearby reaches of the river or the sedimentary rocks that are hydraulically nearby water table beneath the flood plain. This connected to the Colorado River so that water situation occurs in the southeast end of Mohave can move between the river and the aquifer in Valley (pi. 10) where tributary inflow associated response to withdrawal of water from the aquifer with Sacramento Wash (Owen-Joyce, 1987) or differences in water-level elevations between has created a ground-water mound. Although the river and the aquifer. The subsurface limit of tributary inflow is less than 0.1 percent of the the river aquifer is the nearly impermeable annual flow in the river, static water-level bedrock of the bottom and sides of the basins elevations in wells near Sacramento Wash are that underlie the Colorado River valley and tens to hundreds of feet above the river and adjacent tributary valleys, which is a barrier to the accounting surface (fig. 3B). In an area ground-water flow. The accounting surface underlain by a ground-water mound, the static represents the elevation and slope of the water level in a well can remain above the unconfined static water table in the river aquifer accounting surface as long as enough tributary outside the flood plain and the reservoirs of the water can move to the well to replace water Colorado River that would exist if the river were pumped from ground-water storage. If more the only source of water to the river aquifer. The water is pumped from the well than can be accounting surface extends outward from the replaced from a tributary source, static water- edges of the flood plain or a reservoir to the level elevations in the well will decline below subsurface boundary of the river aquifer (fig. 3). the accounting surface and water will eventually The water-table elevation in the river aquifer move toward the well from the river. near a well or well field is assumed to be the Delineation of the subsurface boundaries of the same as the elevation of static water levels in the river aquifer was required prior to the generation wells. This method provides an organized way of the accounting surface in the river aquifer. 6 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water Figure 3. Schematic diagrams showing the river aquifer and accounting surface. Wells labeled "R" have a static water-level elevation equal to or below the accounting surface and are presumed to yield water that will be replaced by water from the river. Wells labeled "T" have a static water-level elevation above the accounting surface and are presumed to yield water that will be replaced by water from precipitation and inflow from tributary valleys. Well labeled "O" is observation well. Concept of the River Aquifer and Accounting Surface 7 DESCRIPTION OF THE RIVER considerable distance beneath the alluvial slopes AQUIFER (Robertson, 1991). Precipitation and inflow from tributary valleys contribute some water to The river and the underlying and adjacent the river aquifer. river aquifer form a complex, hydraulically connected ground-water and surface-water flow system in the lower Colorado River valley from Source of Water in the River Aquifer the east end of Lake Mead to Laguna Dam (fig. 1). Water is stored in three surface The Colorado River is the source for reservoirs and in the river aquifer. Millions of virtually all recharge to the river aquifer in the acre-feet of water are diverted annually from the lower Colorado River valley from Lake Mead to river channel and reservoirs for irrigation of Laguna Dam because about 96 percent of the fields adjacent to the river and for export out of annual water supply to the river valley is from the drainage basin. Much of the irrigation water the river. Surface and subsurface inflows are is transpired by vegetation or evaporates, and the summed for Lake Mead above Hoover Dam and remainder percolates below the root zone into for that part of the river valley between Hoover the river aquifer. Water also infiltrates from the Dam and Laguna Dam. river channel, reservoirs, canals, and marshes For the part of the valley dominated by Lake through the underlying soils and sediments and Mead, the surface inflow from the Colorado recharges the river aquifer. Small quantities of River is the flow at the streamflow-gaging runoff that originate from precipitation infiltrate station above Lake Mead, Colorado River near through the beds of washes and intermittent Grand Canyon, which is 267 miles upstream tributary streams and recharge the river aquifer. from Hoover Dam and outside the study area. Ground water flows downgradient through the The mean annual surface flow is 12,270,000 river aquifer and discharges as seepage into acre-feet for 1923-62 prior to the close of Glen drainage ditches or through the river banks into Canyon Dam (102 miles upstream from the the river. Water moves back and forth between Colorado River near Grand Canyon gaging the surface-water and ground-water systems in station, also outside the study area) and response to application of water to irrigated 10,760,000 acre-feet per year for 1965-90 after fields and annual changes in water levels in the the close of Glen Canyon Dam (Boner and river. Water is pumped from thousands of wells others, 1991). The mean annual inflow into completed in the river aquifer on the flood plain, Lake Mead from the Virgin River as measured at on alluvial slopes, and in tributary valleys. the streamflow-gaging station, Virgin River at Most of the water in the river aquifer Littlefield (fig. 1), is 172,400 acre-feet for originated from the river because of the 1930-90 (Boner and others, 1991). The hydraulic connection to the river and the unmeasured annual tributary inflows to Lake overbank flow that occurred prior to building of Mead are about 156,000 acre-feet (Langbein, the dams. Previously unsaturated sediments and 1954). Thus, the total annual tributary inflow to sedimentary rocks of the river aquifer adjacent to Lake Mead is about 328,400 acre-feet, or about Lakes Mead, Mohave, and Havasu became 3 percent of the total inflow to Lake Mead. saturated with Colorado River water as the For the part of the valley between Hoover reservoirs filled after the dams were closed Dam and Laguna Dam, the surface inflow from (fig. 3A). Ratios of hydrogen and oxygen the Colorado River is the flow below Lake Mead isotopes in ground water from wells in Mohave, as measured at the streamflow-gaging station, Parker, Palo Verde, and Cibola Valleys indicate Colorado River below Hoover Dam (fig. 1). The that most of the water in the river aquifer beneath mean annual flow is about 10,165,000 acre-feet the flood plain and terrace deposits originated for 1935-90 (Boner and others, 1991). Mean from the river and that, in many places, river annual surface and subsurface tributary inflow to water extends from the flood plain for a the river valley between Hoover Dam and 8 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water Imperial Dam is about 144,300 acre-feet lower Colorado River valley are on the surface (Owen-Joyce, 1987), or about 1.4 percent of the of the younger alluvium, and drainage ditches total inflow below Lake Mead. Imperial Dam is and canals are cut into it. Direct runoff from 293 miles downstream from Hoover Dam and occasional intense rainfall infiltrates into this 6 miles upstream from Laguna Dam. Tributary unit in the stream channels of tributaries and inflow between Imperial and Laguna Dams is provides a small quantity of recharge to the river negligible. For the entire study area, the total aquifer. mean annual tributary inflow is about The older alluviums of Miocene, Pliocene, 472,700 acre-feet, or about 4 percent of the total and Pleistocene ages consist of weakly to inflow to the lower Colorado River valley. moderately consolidated gravel, sand, silt, and clay of local origin that were deposited in alluvial fans that extend from the mountains into Geologic Formations and Their the valleys and basins. Local fan gravels Hydrologic Characteristics commonly are interbedded with the rounded gravels, sand, and silt deposited by the ancestral The lower Colorado River flows through a Colorado River (Longwell, 1963; Bales and series of wide alluvial valleys separated by Laney, 1992). The older alluviums overlie the canyons cut into bedrock. In areas outside the Bouse Formation and Muddy Creek Formation flood plain, alluvial slopes rise to mountain and all older rocks. Near Grand Wash (pis. 5-8), ranges that rim the valleys. Sediments and younger basalt underlies or is interbedded with sedimentary rocks that fill the structural basins the older alluviums and is included with the and valleys underlie the flood plain and older alluviums (Bales and Laney, 1992). In reservoirs and extend beneath the alluvial slopes places where the Bouse Formation is absent and of the river valley to the bedrock (fig. 3). The the underlying fanglomerate of local origin river aquifer generally includes, in descending cannot be distinguished from the older order, the younger alluvium, the older alluviums alluviums, the fanglomerate is included with the including the Chemehuevi Formation, the Bouse unit. In this study, the Chemehuevi Formation Formation, and the fanglomerate or the Muddy (Lee, 1908; Longwell, 1936) of Pleistocene age Creek Formation. The consolidated rocks of the is included with the older alluviums. The mountains, referred to in this report as bedrock, Chemehuevi Formation consists of silt, sand, form the bottom and sides of the basins. and gravel that was deposited by the Colorado River from near Lake Mead to as far south as The younger alluvium of Holocene age Chemehuevi Valley and generally is flat lying. consists of unconsolidated gravel, sand, silt, and Where saturated by Lake Mead or Lake Mohave, clay deposited on alluvial slopes and flood plains the unit is a productive aquifer and can yield and in stream channels. The younger alluvium large quantities of water to wells. Potential well was the last unit deposited by the Colorado River yields of the older alluviums range from less as it meandered across the modern flood plain than 100 to more than 1,000 gallons per minute before the dams and diversion structures were and depend on the amount of interbedded built (Metzger and others, 1973). Beneath the flood plain, the younger alluvium is the upper rounded gravels tapped by the well. water-bearing unit of the river aquifer and is The Bouse Formation of Pliocene age from 0 to about 180 feet thick; all but the consists of a thin basal limestone and marl uppermost few feet of the unit is saturated. overlain by clay, silt, and sand (Metzger, 1968; Outside the flood plains of the Colorado and Bill Buising, 1990). The Bouse Formation is a Williams Rivers, the unit generally is above the marine and estuarine formation that was water table and is mapped with the older deposited on fanglomerate and bedrock from alluviums. The unit is highly permeable and can Yum a to as far north as the east bank of Lake yield more than 1,000 gallons per minute of Mohave inT. 23 N., R. 21 W. (G&SR) (pi. 9), in water to wells. Many of the irrigated fields in the Arizona (Bentley, 1979a). At this location, it Description of the River Aquifer 9 interfmgers with the gravel beds of the upper Bohannon, 1984, p. 9) and the conglomerate, unit of the Muddy Creek Formation. The Bouse sandstone, and mudstone of the rocks of the Formation is present in the subsurface and crops Grand Wash trough (Lucchitta, 1966) are out in Mohave and Chemehuevi Valleys; in equivalent to and included with the Muddy Vidal, Chuckwalla, and Smoketree Valleys in Creek Formation. The unit overlies bedrock and California; and in Cactus and La Posa Plains in is widely distributed near Lakes Mead and Arizona. Clays and silts of the lower part of the Mohave and in the Moapa and Virgin Valleys Bouse Formation are almost impermeable and (pis. 1-7). The conglomerate is the most widely can confine water in the underlying distributed facies of the Muddy Creek Formation fanglomerate. Upper sandy layers of the Bouse in the Lake Mead area; the formation consists Formation are moderately permeable and can mainly of conglomerate near Lake Mohave. The yield a few hundreds of gallons per minute of conglomerate facies probably will yield a few to water to wells. as much as 100 gallons per minute of water to wells. A mudstone facies that contains gypsum The fanglomerate of Miocene and Pliocene and halite is present in much of the area near ages consists of moderately to firmly cemented Lake Mead. The mudstone facies is nearly continental sandy gravel of local origin that impermeable and will not yield significant overlies bedrock and underlies the Bouse quantities of water to wells. Formation (Metzger and others, 1973). Where the Bouse Formation is absent and the The bedrock consists of volcanic and fanglomerate cannot be reliably differentiated, sedimentary rocks of Mesozoic and Tertiary it is assigned to the older alluviums. In ages and igneous, metamorphic, and Chemehuevi Valley, late Tertiary sedimentary sedimentary rocks of Precambrian, Paleozoic, rocks yield water to wells at Crystal Beach and Mesozoic ages. Bedrock consists of all rocks (pi. 10). These rocks and the fanglomerate are older than the fanglomerate and Muddy Creek included with the older alluviums in this area. In Formation and is equivalent to the bedrock the Chocolate and Laguna Mountains near defined by Metzger and others (1973). These Laguna Dam, the conglomerate of Chocolate rocks are dense, consolidated, and weakly to Mountains and the upper member of the Kinter firmly cemented. The bedrock is nearly Formation (Olmsted and others, 1973) are impermeable but probably will yield a few included with the fanglomerate (pi. 19). The gallons per minute of water to wells where it is fanglomerate is recognized from Yuma to as far fractured or weathered. Some of the volcanic north as the point where the Bouse Formation flows and sedimentary rocks of Tertiary age will interflngers with the Muddy Creek Formation yield a few tens of gallons per minute to wells. east of Lake Mohave. North of this point, the conglomerate of the Muddy Creek Formation is equivalent to the fanglomerate. The Delineation of the River-Aquifer fanglomerate is fine grained and contains Boundary gypsum layers and crystals beneath La Posa Plain (pi. 15). The fanglomerate probably will The river-aquifer boundary was delineated yield a few to a few hundreds of gallons per primarily on information from previously minute of water to wells where it is composed published geologic, hydrologic, and geophysical mainly of sand and gravel. studies. Areal extent, saturated thickness below The Muddy Creek Formation (Longwell, river level, and subsurface continuity of 1928, 1936; Bohannon, 1984) of late Miocene to sediments and sedimentary rocks that form the Pliocene age consists of moderately to firmly river aquifer were inferred from hydrologic, cemented continental gravel, sand, silt, clay, geologic, and geophysical maps and studies and gypsum, and halite of local origin that is lithologic, geophysical, and drillers' logs of interbedded with basalt flows. The Hualapai wells. Extent and thickness of low-density Limestone (Blair and Armstrong, 1979; sediments that were assumed to form the river 10 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water aquifer in several areas were determined by The river aquifer extends from the geophysical-gravity studies done during this flood plain to an intersection with investigation (see section entitled "Gravity bedrock. Studies"). The position of the river-aquifer Saturation and hydraulic connection boundary shown on plates 1-19 is intended to be exist in the river aquifer where several directly above the subsurface intersection of the hundred feet of sediments and sedi accounting surface and the bedrock surface mentary rocks are present below river (fig. 3). The position is approximate in much of level between the flood plain and the the study area because subsurface data from bedrock. boreholes or geophysical studies commonly are not available near the edge of the river aquifer. Static water-level elevations in wells on The boundary generally was drawn near the the alluvial slopes and in adjacent surface contact between the sediments and tributary valleys provide local values sedimentary rocks and the bedrock unless of the elevation of the water table and subsurface data were available to better define indirect evidence of hydraulic con the position. The river-aquifer boundary was nection to the flood plain where delineated on the basis of the following sufficient wells are available to define scientific assumptions: the water table. The younger alluvium, the older allu The river aquifer is present in five reaches of viums, the sandy part of the Bouse the lower Colorado River valley from the east Formation, the fanglomerate, and the end of Lake Mead to Laguna Dam. The reaches Muddy Creek Formation including are above Davis Dam, Davis Dam to Topock, the rocks of the Grand Wash trough, Topock to Parker Dam, Parker Dam to Draper collectively, are permeable, hydrauli- Lake, and Clear Lake to Laguna Dam. Draper cally connected, store and transmit and Clear Lakes are flooded depressions in the significant quantities of water, and flood plain that are connected to the river. The river-aquifer boundary was not delineated form an aquifer. between Draper and Clear Lakes because no Mountain masses and basin rims of significant amount of saturated sediments and bedrock are effective barriers to sedimentary rocks are present outside the flood ground-water flow; interbasin flow plain. through mountain ranges is negligible in relation to the magnitude of recharge Above Davis Dam from the Colorado River. After Hoover Dam was closed in 1935, the The position of the river-aquifer previously dry sediments and sedimentary rocks boundary generally is a few to a few above the level of the Colorado, Virgin, and thousands of feet toward the river from Muddy Rivers became saturated with river the contact between the alluvial slopes water as Lake Mead filled. The maximum and the bedrock because the slopes are water-surface elevation of Lake Mead was underlain by bedrock near the 1,225.85 feet in 1983 (Boner and others, 1991). mountains. A small quantity of water that originated from the Colorado River is stored in the younger For the purpose of the gravity studies, alluvium near the point where the Virgin and low-density sediments that fill struc Muddy Rivers enter Lake Mead and near the lake tural basins between mountains are shore in Detrital Wash, Las Vegas Wash, and equivalent to the sediments and Grand Wash (pi. 5). sedimentary rocks that form the river Large volumes of the river aquifer are aquifer. present beneath the lake and adjacent to parts of Description of the River Aquifer 11 the shore line around Lake Mead (pis. 1-8). The depression on the basis of thickness maps of the lake extends for about 105 miles along the Muddy Creek Formation, gravity maps, and Colorado River from Hoover Dam to the Grand geologic maps (Blank and Kucks, 1989; Wash Cliffs and about 60 miles up the canyon of Bohannon and others, 1993). The boundary the Virgin and Muddy Rivers. Most of the lines were extended south along each side of the younger alluvium that forms the flood plain of Overton Arm of Lake Mead (pi. 5) on the basis the Colorado River and Virgin River is now of geohydrologic maps (Bales and Laney, 1992; submerged beneath the water of Lake Mead. J.T. Bales and R.L. Laney, written commun., Older alluviums and the underlying conglo 1993). merate facies of the Muddy Creek Formation are Significant volumes of the river aquifer the main water-bearing units of the river aquifer. extend beneath and are adjacent to Lake Mohave Hualapai Limestone caps the Muddy Creek for about 34 miles from the end of the bedrock Formation and rocks of the Grand Wash trough canyon below Hoover Dam to the beginning of and is above lake level in most places. The the bedrock canyon above Davis Dam (pis. 7 Hualapai Limestone can be water bearing where and 9). The younger alluvium of the flood plain it is below lake level in T. 31 N., R. 17 W. was covered with river water, and previously dry (G&SR), at Sandy Point in Arizona (pi. 5). sediments and sedimentary rocks became The Grand Wash trough is a structural basin saturated to an elevation of 645 feet as the adjacent to the Grand Wash Cliffs that extends reservoir filled behind Davis Dam. The river- from southwest to northeast across the east end aquifer boundary was drawn near the contact of Lake Mead (pis. 5-8). The trough is partly between the bedrock and the sediments and filled with conglomerate, mudstone, limestone, sedimentary rocks around the rim of the basin and basalt flows from below lake level to as and around several isolated hills of bedrock. much as 2,200 feet above lake level. The river- aquifer boundary was drawn from the lake shore Davis Dam to Topock along each side of the trough and across the trough at the location where bedrock appeared to Mohave Valley below Davis Dam is a trough be above lake level. rimmed by the bedrock of the Dead Mountains The Virgin River depression is a deep on the west and the Black Mountains on the east structural basin that underlies the lower Virgin (pis. 9 and 10). The river enters the valley in a and Muddy Rivers in the Moapa and Virgin bedrock canyon below Davis Dam and exits the Valleys in Nevada, Arizona, and Utah valley into a bedrock canyon at Topock. The (Bohannon and others, 1993). The depression is river aquifer is adjacent to the channel or the rimmed by the Muddy, Virgin, Beaver Dam, and flood plain of the Colorado River for about Mormon Mountains and the Tule Spring Hills 40 miles from Bullhead City to the end of the (pis. 1-4). Geologic and gravity data (Blank and valley at Topock. The river flows in a narrow Kucks, 1989) and seismic and gravity studies channel cut into the older alluviums for about (Bohannon and others, 1993) indicate that much 10 miles below Davis Dam to Big Bend. Below of the basin contains at least 3,000 feet of Muddy Big Bend, the flood plain widens and ranges Creek Formation that is underlain by 0 to as from 2 to 5 miles wide for 30 miles to Topock. much as 15,000 feet of sedimentary rocks of The river aquifer in Mohave Valley consists Tertiary age. A test well (Mobile Virgin 1A) on of the younger alluvium, the older alluviums, the Mormon Mesa penetrated 2,201 feet of siltstone Bouse Formation, and the fanglomerate; the and 699 feet of gypsum and gypsiferous siltstone main water-bearing units outside the flood plain of the Muddy Creek Formation. Geologic and are the older alluviums and the fanglomerate. geophysical data from the test well provided The combined thickness of the units ranges from calibration for the geophysical studies that 0 to more than 5,000 feet in the central part of the defined the depression. The river-aquifer valley (Freethey and others, 1986). The younger boundary was drawn near the rim of the alluvium was deposited in a trough cut into the 12 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water older alluviums by the Colorado River at its and beneath Lake Havasu for about 30 miles lowest degradation and ranges from 0 to more from the end of the bedrock canyon below than 167 feet thick beneath the flood plain. The Topock to the mouth of Bill Williams River beds of many of the larger washes outside the (pis. 10 and 12). After Parker Dam was closed, flood plain are underlain by a thin layer of the the water in Lake Havasu rose to an elevation of younger alluvium, which is mapped with the 449.6 feet and saturated as much as 60 feet of older alluviums. The older alluviums extend previously dry sediments and sedimentary rocks. from the foot of the bedrock mountain ridges down the slopes to the edge of the flood plain The older alluviums, the fanglomerate, and and underlie the younger alluvium beneath the the underlying sedimentary rocks of Tertiary age flood plain. The older alluviums are from 0 to form the river aquifer at Crystal Beach, in the more than 600 feet thick near Bullhead City, Lake Havasu City area, and in a valley southeast more than 350 feet thick at the Mohave Power of Lake Havasu City. Combined thickness of the Plant southwest of Laughlin, 350 feet thick near units is as much as 1,000 feet at Lake Havasu Big Bend, and 200 feet thick near Topock (pis. 9 City. The older alluviums and the fanglomerate and 10). The Bouse Formation underlies the form the river aquifer in and near the older alluviums in much of Mohave Valley and is as much as 254 feet thick. The Bouse Chemehuevi Indian Reservation west of the Formation is absent in an area near Big Bend lake and north of Chemehuevi Wash. The where older alluviums overlie fanglomerate fanglomerate forms the river aquifer west of the (pi. 9). Fanglomerate underlies the Bouse lake and south of Chemehuevi Wash where the Formation and crops out west of the river near contact between the base of the Bouse Formation Big Bend and at isolated points around the rim of and the fanglomerate is exposed in a few square the valley at the toe of the slope of the miles (pi. 12). At the southeast end of Lake mountains. Sacramento Wash enters the Havasu, sedimentary breccia and sandstone of southeast end of Mohave Valley where as much Miocene age (Sherrod, 1988) are included with as 1,000 feet of the older alluviums, Bouse the fanglomerate (pi. 12). These units are Formation, and fanglomerate are present in the present beneath the flood plain of the Bill gap between Mohave and Sacramento Valleys Williams River, crop out along its south bank (pi. 10). upstream from Lake Havasu, and extend beneath The river-aquifer boundary was drawn near basalt flows of the Buckskin Mountains. the contact between the bedrock and the The river-aquifer boundary generally was sediments and sedimentary rocks around the rim of the valley except at Big Bend and Sacramento drawn near the contact between older alluviums Wash (pi. 10). The river-aquifer boundary was and bedrock. At Crystal Beach and the mouth of drawn at the edge of the flood plain at the contact the bedrock canyon below Topock, the contact with bedrock along the west side of Big Bend. In was drawn between sedimentary rocks of the southeastern part of the valley, the boundary Tertiary age and consolidated rocks. The was drawn across Sacramento Wash west of a boundary was drawn along the southwest side of bedrock outcrop near the wash, along the the Mohave Mountains to the Bill Williams southwest side of the Buck Mountains, across River, along the north bank of the Bill Williams the basin to the Mohave Mountains, and along River near the contact of the fanglomerate and the Mohave Mountains to Topock. bedrock, across the river at the mouth of a bedrock canyon, and near the edge of the Topock to Parker Dam Buckskin Mountains where the fanglomerate pinches out beneath basalt flows. The boundary The Colorado River flows in a bedrock was drawn near the bedrock contact along the canyon below Topock to the upper end of Lake west side of the valley except for two gaps where Havasu. The river aquifer is present adjacent to the bedrock is concealed. Description of the River Aquifer 13 Parker Dam to Draper Lake of the fanglomerate; 2,360 feet of these rocks were penetrated in a test hole in section 24, Parker, Palo Verde, and Cibola Valleys are T. 7 N., R. 19 W. (G&SR). From Parker to parts of a reach of the river that extends about south of Quartzite, Arizona, the Bouse 80 miles from Parker Dam sited in a bedrock Formation is continuous in the subsurface. At a canyon to a bedrock canyon downstream from site in section 34, T. 8 N., R. 19 W. (G&SR), the Draper Lake (pis. 11-18). Geohydrologic Bouse Formation is at least 1,000 feet thick and studies (Metzger and Loeltz, 1973; Metzger and yielded 250 gallons per minute of water to a test others, 1973; Freethey and others, 1986), well. In T. 7 N., R. 19 W. (G&SR), the Bouse drill-hole data, and geophysical data indicate Formation is 520 to 590 feet thick; near that the flood plain, alluvial slopes outside the Quartzite, the unit is as much as 800 feet thick flood plain, and several adjacent alluvial valleys and consists mainly of silt and clay. At a site in are underlain by thick sections of the river section 10, T. 3 N., R. 19 W. (G&SR), the Bouse aquifer. In much of the reach, the flood plain is Formation is more than 529 feet thick and is from 3 to 9 miles wide and is underlain by described as "mudstone" that consists mainly of younger alluvium to a depth of about 180 feet. sandy, silty clay. Adjacent alluvial slopes and tributary valleys extend from the flood plain more than 46 miles The river-aquifer boundary was drawn along west into California and more than 37 miles the east and west edges of La Posa Plain to about southeast into Arizona. 3 miles north of Quartzite near the probable location where the subsurface contact between The river-aquifer boundary was drawn from the older alluviums and the Bouse Formation is the canyon below Parker Dam, along the bedrock above the water table (see dotted line on outcrop of the Buckskin Mountains northeast of plate 15). The Bouse Formation and the Parker, around the rim of Cactus Plain to where underlying fine-grained fanglomerate form a Bouse Wash enters the plain, and across the barrier to the movement of significant quantities north end of the Plomosa Mountains to the La of ground water. Drillers' logs of wells near Posa Plain (pis. 12, 13, 15, and 16). At Parker, Quartzite indicate that the Bouse Formation the fanglomerate yields a few hundreds of yields only a few gallons per minute of water to gallons per minute of water to wells. Black Peak wells. Beneath La Posa Plain, the fanglomerate and Mesquite Mountain are hills of bedrock that probably will not yield significant quantities of are surrounded by the river aquifer. The river water because it is fine grained and contains aquifer is continuous in the subsurface from La layers and zones of gypsum. Posa Plain between Mesquite Mountain and Moon Mountain to the flood plain on the basis of The river aquifer is present beneath the a gravity study (see section entitled "Gravity surface of the alluvial slopes from the flood plain Studies"). Older alluviums cover most of the southwest of Parker, west to near Vidal, and terraces, alluvial slopes, and floors of adjacent southwest to the Riverside Mountains (pi. 12). alluvial valleys. In the older alluviums, surface The river-aquifer boundary was drawn along the and subsurface layers of rounded gravels axis of buried ridges of bedrock between the deposited by the Colorado River are interbedded Whipple and Riverside Mountains; the position with material of local origin; the interbedding of the axis was determined by a gravity study indicates that the ancestral Colorado River (see section entitled "Gravity Studies"). The flowed south from near Parker across Cactus and most probable position of the river-aquifer La Posa Plains to the east of Mesquite Mountain boundary between Vidal and the Riverside and to the southwest between Mesquite Mountains is along the axis of the buried ridge. Mountain and Moon Mountain (pis. 12 and 15). This position is supported by static water-level Cactus and La Posa Plains east and southeast of elevations at Vidal, which are about 57 to 60 feet Parker in Arizona (pis. 12 and 15) are underlain above the accounting surface. The boundary was by as much as 780 feet of the older alluviums, drawn northeast to the bedrock of the Whipple 1,000 feet of the Bouse Formation, and 990 feet Mountains and then along the southeast edge of 14 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water those mountains to the canyon below Parker Ground to the Laguna and Muggins Mountains Dam. (pi. 19) on the basis of drillers' logs, outcrop The river aquifer is present beneath Palo patterns, and estimated depths to bedrock in a Verde Mesa from the edge of the flood plain gravity study by Oppenheimer and Sumner west to the McCoy, Mule, and Palo Verde (1981). Hydraulic connection between the river, Mountains (pis. 15 and 18). Hydraulic connec including Martinez Lake, and the river aquifer tion to the flood plain is indicated by static in the trough is inferred from water-level water-level elevations in wells and water-surface elevations in wells. The river aquifer is elevations in drainage ditches near the edge of separated from the river by bedrock of a part of the flood plain (Owen-Joyce, 1984, p. 16). The the Chocolate Mountains (also known locally as river-aquifer boundary was drawn near the Laguna Mountains) from about 1 mile south of bedrock outcrop of the mountains. Martinez Lake to a gap between the Chocolate Mountains and the Laguna Mountains 2 miles The river aquifer is continuous in the southeast of Yuma Test Station. The river- subsurface between Chuckwalla Valley and Palo aquifer boundary was drawn near the contact Verde Mesa through the narrows near Interstate between the older alluviums and the bedrock in 10 on the basis of a gravity study (see section most of the area. entitled "Gravity Studies"), a seismic profile, and drillers' logs (Metzger and others, 1973). The river-aquifer boundary was drawn Static water-level elevations indicate hydraulic across a valley between the bedrock of the Castle connection because the water table slopes Dome and Muggins Mountains near a probable toward the Colorado River at about 1.3 feet per ground-water divide about 17 miles east of the mile from the eastern part of Chuckwalla Valley river (see dotted line on plate 19). A particle- through the narrows to Palo Verde Mesa. The size log of a nearby well indicates that sediments bottom of the basin is below sea level from the and sedimentary rocks are present to a depth of east end of the valley to 6 miles northeast of about 700 feet, which is about 130 feet below the Desert Center (pi. 14). Subsurface continuity of accounting surface. The position of the well, the river aquifer and hydraulic connection from which has a reported static water-level elevation the northwestern part to the eastern part of of 65 feet above the accounting surface, the valley are inferred from drillers' logs and indicates that the well is near a probable ground- static water-level elevations. The river-aquifer water divide. The river-aquifer boundary was boundary was drawn near the bedrock outcrop drawn across a 3-mile-wide gap between the around the rim of the valley (pis. 11, 14, 15, Laguna and Muggins Mountains 1 mile north of and 17). the edge of the Gila River flood plain (see dotted The river aquifer extends from the flood line on plate 19). The gravity study by plain beneath Milpitas Wash and into Smoketree Oppenheimer and Sumner (1981) indicates that Valley (pi. 18). More than 2,100 feet of older bedrock probably is within a few hundred feet of the surface. alluviums, Bouse Formation, and fanglomerate are present near the mouth of the valley on the basis of a gravity study (see section entitled "Gravity Studies"), a seismic profile, and data Gravity Studies from an oil test hole (Metzger and others, 1973). The river-aquifer boundary was drawn near the By D. R. Pool bedrock outcrop around the rim of the valley. Gravity studies were used to help delineate Clear Lake to Laguna Dam the extent and thickness of the river aquifer by estimating the thickness of low-density The river aquifer is present in a trough east sediments (sediments and sedimentary rocks) in of the river that extends from Clear Lake in a a few areas where the river valley has a surficial southeasterly direction through Yuma Proving connection with a tributary valley (fig. 1). The Description of the River Aquifer 15 subsurface configuration of the sides and using three different radial zones around each bottoms of the structural basins that contain the station. Gravitational effects of terrain beyond river aquifer and limit ground-water flow is 6,562 feet and Earth curvature beyond 8.7 miles poorly known. The presence of shallow bedrock were calculated using digital-terrain data on above river level in these areas of surface 15-second, 1-minute, and 3-minute grids and the connection could limit the extent of the river program BOUGUER (Godson and Plouff, 1988). aquifer because the bedrock would be a barrier to Effects of terrain between 148 and 6,562 feet the flow of ground water between the river were calculated using 7.5-minute digital- valley and the tributary valley. The presence of elevation models that were available for most of a significant thickness of low-density sediments the area and the program TCINNER (Cogbill, below the level of the Colorado River would 1990). The effects of local terrain within indicate that the river aquifer and therefore the 148 feet were calculated from field notes and accounting surface should extend into the topographic maps using a sloping-wedge tributary valley. River-aquifer thickness can be technique (Barrows and Fett, 1991). estimated by using gravity methods because Patterns of low-gravity values in the areas of gravity values are inversely related to the interest can be simulated as resulting from a thickness of low-density sediments and because thickness of low-density rocks using theoretical- the river aquifer primarily consists of permeable gravity models, provided that the subsurface sediments that are much lower in density than geology is not complex. Gravity models of the the nearly impermeable bedrock. Gravity values subsurface geology were constructed for each of at the greatest thickness of low-density the areas of interest, with the exception of Rice sediments are low relative to gravity values Valley, to simulate the thickness and extent of where bedrock is shallow. Tributary valleys of low-density sediments. A two and one-half- interest include La Posa Plain, Vidal Valley, dimensional gravity model, SAKI (Webring, Rice Valley, Chuckwalla Valley, and Smoketree 1985), was used to simulate the gravity Valley. The older alluviums, the Bouse distribution along profiles in each area. A model Formation, and the fanglomerate crop out at the for the area of Rice Valley was not necessary surface in or near these five tributary valleys; because field reconnaissance and geologic maps drill-hole data provide evidence of the presence revealed the presence of bedrock outcrops that of these low-density sediments in the subsurface. eliminated the possibility of a hydraulic The connection between the tributary valleys connection between the aquifer in Rice Valley and the lower Colorado River valley commonly and the river aquifer. includes a buried bedrock ridge composed of dense crystalline rock. Reliable estimates of thickness of low- density sediments from the gravity-model Gravity measurements were made at more simulations were dependent on information that than 600 stations and added to an existing data could be used to calibrate the subsurface-density base for the area of the lower Colorado River distribution. Calibration information includes (Mariano and others, 1986). Data were collected data from wells and subsurface geophysics that in each area of interest with the exception of define the density or thickness of low-density Chuckwalla Valley where the existing data rocks near the area of interest. Calibration base was sufficient for analysis. The Global information for gravity models in the study area Positioning System (Remondi, 1985) was used included several deep wells that penetrate to survey the latitude, longitude, and elevation of bedrock and some seismic data. Gravity profiles the new gravity stations. The newly collected were completed in each of the areas where gravity data were reduced to complete-Bouguer bedrock control was available to calibrate the anomaly values and merged with the existing density of subsurface geologic units in the area. U.S. Geological Survey gravity data base. The The calibrations resulted in a consistent set of data were adjusted for the effects of terrain and density values for the low-density sediments that Earth curvature to a distance of 103.6 miles was common to all areas and was used for each 16 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water simulated gravity profile. Densities of 1.90, Mountains (fig. 4). The calibration profile 2.10, 1.90, 2.10, 2.00, and 2.30 grams per cubic extended from Mesquite Mountain southeast to centimeter were used to simulate unsaturated the Plomosa Mountains through the two test alluvium, saturated alluvium, unsaturated holes. The elevation of the bedrock surface is Bouse Formation, saturated Bouse Formation, more than 1,475 feet below sea level at a test unsaturated fanglomerate, and saturated fan- hole in the northwest quarter of section 24, glomerate, respectively. Metamorphic and T. 7S., R. 19 W (G&SR). The gravity dis granitic rocks underlying the low-density tribution along a west-east profile extending sediments were assumed to have a density of from the Colorado River valley to the Plomosa 2.67 grams per cubic centimeter. Volcanic Mountains was simulated to estimate the rocks were simulated using densities of 2.42 or thickness of low-density sediments at the buried 2.50 grams per cubic centimeter. ridge between the Dome Rock Mountains and Mesquite Mountain (fig. 4, A-A'}. The density La Posa Plain distribution of the sediments mentioned above resulted in close agreement between simulated A gravity study indicates that low-density bedrock elevation and the bedrock elevation at sediments of the river aquifer are continuous in the test holes along the calibration profile. the subsurface below river level from Parker Valley along the south side of Mesquite The position of the west-east profile was Mountain into La Posa Plain and that depth to selected to include a well at the southwest corner bedrock below river level exceeds 330 feet of section 28, T. 7 S., R. 19 W. (G&SR), which (fig. 4 and pi. 15). A strong gravity gradient has a water level above the level of the river. No shown by closely spaced gravity-anomaly depth or lithologic information is available for contour lines south of Mesquite Mountain indi the well, but the water level at the well indicates cates a large thickness of low-density sediments that the river aquifer is present near the level of in the area south of Mesquite Mountain. The the river at the site. A planar-regional gravity complete-Bouguer gravity anomaly values also trend was removed from the complete-Bouguer indicate an arcuate-shaped buried bedrock ridge gravity values in the area of the profile to create extending from the Dome Rock Mountains to the residual-gravity profile simulated by the near a test hole and an adjacent bedrock low model. The planar trend was calculated on the that lies to the northwest. The structures are basis of gravity values on bedrock at the north commensurate with antiformal and synformal end of the Dome Rock Mountains, at Mesquite structures, respectively, mapped by Scarborough Mountain, and at the Plomosa Mountains. The (1985). The gravity signature of the ridge is resulting gravity model simulated maximum obscured by a strong regional gravity trend from elevation of the bedrock surface as 30 feet above low values (-50 to -42 milligals) in the Dome sea level and about 330 feet below river level .Rock Mountains to higher values near Mesquite where the profile crosses the trace of the buried Mountain (-26 milligals) and the Plomosa bedrock ridge about 5,000 feet west of the well. Mountains (-40 to -32 milligals). Elevation of the bedrock surface at the well was Two-dimensional gravity models were simulated at more than 500 feet below sea level. constructed along profiles across La Posa Plain The elevation of the bedrock surface probably is for purposes of calibrating density values of lower by several hundreds of feet between geologic units and estimating the thickness of the structural high along the profile and low-density sediments between the Dome Rock Mesquite Mountain. Model results indicate that Mountains and Mesquite Mountain (fig. 5). The substantial thicknesses of low-density sediments distribution of complete-Bouguer gravity anom could be present between the Dome Rock aly values and the data from two test holes in La Mountains and Mesquite Mountain but probably Posa Plain indicate a deep structural trough are restricted to the area of the synformal between Mesquite Mountain and the Plomosa structure south of Mesquite Mountain. Description of the River Aquifer 17 114°00' a o a I 5' a I 0) i ? BOUNDARY \ OF FLOOD + PLAIN Io 3D 33°45 Profile shown in figure 5 5 MILES i i i i | 5 KILOMETERS EXPLANATION A' SEDIMENTS AND SEDIMENTARY BURIED RIDGE TRACE OF MODELED-GRAVITY PROFILE ROCKS SYNFORMAL STRUCTURE - -54 - LINE OF EQUAL GRAVITY ANOMALY Interval BEDROCK 2 milligal ANTIFORMAL STRUCTURE + GRAVITY STATION PROBABLE LOW-DENSITY SEDIMENTS BELOW RIVER LEVEL Figure 4. Complete-Bouguer gravity anomaly for La Posa Plain, Arizona (geologic structures from Scarborough, 1985). Plomosa Mountains »'_.-. .* '.- v '.* * * Unsaturated alluvium I Density = 1.90 sturaied Bowse ? V Tv Saturated Bouse Formation . f I. Density = 2,10 Saturate fangiom^rate 2.30 *L >* Metamorphic rock Density = 2.67 -3,500 I I "" I * I 14 16 18 DISTANCE, IN MILES VERTICAL EXAGGERATION x 6 /1 A 5 1 1 1 1 1 1 1 1 0 - A OBSERVED A - + SIMULATED -5 -10 AA ^ t AAA^ ^ -15 -20 1 1 1 1 1 1 1 1 () 2 4 6 8 10 12 14 16 1 DISTANCE, IN MILES Figure 5. Observed and simulated two-dimensional gravity model for La Posa Plain, Arizona. A, Gravity model. B, Observed and simulated gravity profile. Vidal Valley ^ - where the thickness of low-density sediments A gravity study indicates that buried ridges of below river level is minimal. The distribution of bedrock are present in the subsurface below river complete-Bouguer gravity anomaly values in Vidal level between the Riverside Mountains and the Valley reflects the primary structural trends in the Whipple Mountains in Vidal Valley (fig. 6). The crystalline rocks in the area. Northeast-trending most probable position of the river-aquifer antiforms and synforms are crossed by northwest- boundary is along the axis of the buried ridges trending normal-faulted and rotated blocks (Davis Description of the River Aquifer 19 3- o Q. a I I 3T I i a f y 30 5*§ a.8 BOUNDARY OF FLOOD PLAIN o 34°00' L I'30 5 MILES 5 KILOMETERS i EXPLANATION SEDIMENTS AND SEDIMENTARY BURIED RIDGE -T- NORMAL FAULT Ball on downthrown side. ROCKS Dashed where approximately located J SYNFORMALSTRUCTURE BEDROCK - -40 - LINE OF EQUAL GRAVITY ANOMALY Interval ANTIFORMAL STRUCTURE 2 milligal POSSIBLE LOW-DENSITY SEDI + GRAVITY STATION MENTS BELOW RIVER LEVEL Figure 6. Complete-Bouguer gravity anomaly for Vidal, California (geologic structures from Davis and Anderson, 1991; Carr, 1991). and Anderson, 1991; Carr, 1991). The normal- well in the northeast quarter of section 14, faulted blocks are tilted to the southwest resulting in T. 7 S., R. 21 E. (SB), that did not penetrate gravity highs at the northeast at the upthrown edge bedrock but represents the minimum thickness of the block and gravity lows at the southwest at the of low-density sediments. Seismic data along downthrown edge of the block. A prominent the seismic-gravity profile of Metzger and others northeast-trending gravity high is present along the (1973) indicated the presence of a substantial southwest projection of an antiformal structure in thickness of river aquifer about 1,250 feet the Whipple Mountains. An outcrop of volcanic because the top of bedrock was interpreted to be rock on the upthrown side of a prominent normal about 1,000 feet below sea level. Gravity data fault lies at the southwest extent of the gravity high analyzed in this study indicate that the buried near Vidal, California. The northeast-trending ridge is about 10,000 feet east of the gravity high indicates the presence of a major seismic-gravity profile of Metzger and others buried ridge that restricts the thickness of low- (1973; fig. 7, this report). The two and one-half- density sediments between Vidal Valley and the dimensional gravity model (fig. 8) indicates that Colorado River valley. Low-density sediments the river-aquifer thickness is much more could be present below river level along the buried restricted at the buried ridge than along the ridge in the areas between the two southwest-most seismic-gravity profile of Metzger and others normal faults and between the outcrop of volcanic (1973). The top of the bedrock surface was rocks near Vidal and the Riverside Mountains. The simulated to be about 330 feet below sea level. area between the two normal faults, however, The saturated alluvial deposits were simulated probably does not have a substantial thickness of as 3 miles wide at the buried ridge; the low-density sediments below river level because of fanglomerate was simulated as 1.9 miles wide the lack of a significant gravity low in the area. A at the buried ridge. The simulated bedrock two-dimensional gravity model indicated that a elevation at the intersection of the gravity profile substantial thickness of low-density sediments or and the previous seismic-gravity profile was intermediate-density volcanic rocks of the bedrock 500-650 feet below sea level, which is similar to could be present below river level between Vidal the results of the previous investigation by and the Riverside Mountains; however, subsurface Metzger and others (1973). lithologic information is insufficient to determine the rock type. Smoketree Valley Chuckwalla Valley A gravity study indicates that low-density sediments of the river aquifer are present in the A gravity study indicates that low-density subsurface below river level in Smoketree sediments of the river aquifer are continuous Valley and the distribution of complete-Bouguer from Palo Verde Mesa into Chuckwalla Valley gravity anomaly values between the Midway and that depth to bedrock below river level Mountains and Palo Verde Mountains indicates exceeds 560 feet. A two and one-half- that no buried bedrock ridge separates dimensional gravity model was completed for Smoketree Valley from the Colorado River the area where Chuckwalla Valley adjoins Palo valley (fig. 9). A deep trough that contains low- Verde Mesa using available isostatic-residual density sediments and intermediate-density gravity anomaly data (fig. 7, B-B'). A modeled volcanic rocks is present on the basis of the gravity profile was oriented west to east across gravity, seismic, and lithologic data from an oil an apparent buried bedrock ridge between the test hole and a deep well. A two-dimensional McCoy Mountains and Mule Mountains that gravity model was constructed to simulate the could restrict the river-aquifer thickness. The thickness of low-density sediments along a modeled gravity profile crossed an area where north-south profile through the area (fig. 9, depth to bedrock had previously been estimated C-C"; fig. 10). The profile lies along the transect using seismic and gravity data along a north- of a previously existing seismic profile (Metzger south profile (Metzger and others, 1973, p. 42). and others, 1973). Results of the simulation are The modeled profile included data from a deep consistent with data from the oil test hole and Description off the River Aquifer 21 114°30' 10 10 s*a a § I s Q.I f Ehrenberg BOUNDARY OF / FLOOD PLAIN o 33°30' 30 Profile shown in figure 8 5 MILES Gravity-anomaly patterns modified S from Mariano and others, 1986 f 0 5 KILOMETERS (D EXPLANATION SEDIMENTS AND SEDIMENTARY BURIED RIDGE - -20 - LINE OF EQUAL GRAVITY ROCKS B B 1 ANOMALY Interval 2 TRACE OF MODELED-GRAVITY PROFILE milligal BEDROCK TRACE OF SEISMIC AND GRAVITY + GRAVITY STATION PROFILE (from Metzger and others, 1973) Figure 7. Isostatic-residual gravity anomaly for Chuckwalla Valley, California. Saturated Bouse Formation < i Saturate^ aiiuvium Density = 2.10.' : ( » Denjsiiy =5 2.1 (T ^.;^>y^ ^ » Saturated Souse Formation Saturated langiomerate Density « 2.30 - Saturated Janglomerale Density i 2.30 ; Metamorphic rock Density = 2.67 10 12 14 16 DISTANCE, IN MILES VERTICAL EXAGGERATION x 6 B B' 1 1 1 1 1 1 1 toMio-L A OBSERVED A A ^ A A ^ >^ . A RESIDUALGRAVITY own ANOMALY,INMILLIGALS i - + SIMULATED ^ T ^ M - * 4 * 4 4 _in ?A _ i i i i i i i 6 8 10 12 14 16 DISTANCE, IN MILES Figure 8. Observed and simulated two and one-half-dimensional gravity model for Chuckwalla Valley, California. A, Gravity model. B, Observed and simulated gravity profile. Description of the River Aquifer 23 114°40' ' I BOUNDARY OF \ FLOOD PLAIN---""" ) 10' 33°05' Profile shown in figure 10 5 MILES I i i i i I 0 5 KILOMETERS EXPLANATION c c SEDIMENTS AND SEDIMENTARY TRACE OF MODELED-GRAVITY ROCKS PROFILE -54 BEDROCK LINE OF EQUAL GRAVITY ANOMALY Interval 2 milligal GRAVITY STATION Figure 9. Complete-Bouguer gravity anomaly for Smoketree Valley, California. 24 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water Unsaturated sediments Saturated Volcanic Density = Bouse rock 1.90-2.00 Formation Density = 2 42 Density = 2.10 Saturated fanglomerale Density 10 DISTANCE, IN MILES VERTICAL EXAGGERATION x 6 B C GRAVITY MILLIGALS -40 A* jCAAf A i> -45^ u i . _ z> > -50 ^s. Q -! ^^h44. A ^ OBSERVED z -55 + SIMULATED Kn 1 1 1 1 10 DISTANCE, IN MILES Figure 10. Observed and simulated two-dimensional gravity model for Smoketree Valley, California. A, Gravity model. B, Observed and simulated gravity profile. Description of the River Aquifer 25 seismic data. The base of the fanglomerate was regulated flow conditions of the late 1980's and simulated to be about 1,540 feet below sea level early 1990's, delivery of full allocations of at the oil test hole. The base of the fanglomerate Colorado River water to users in the United is at a much lower elevation north of the oil test States, and river-channel conditions surveyed hole and south of Milpitas Wash. The gravity between 1980 and 1988. The water-surface model and the seismic data indicate that a steeply profiles were computed for the highest median dipping interface, possibly a normal fault, is monthly projected discharge in the Colorado present between low-density sediments and River for 1992-2001. The Bureau of high-density rocks near Milpitas Wash. Reclamation determined the discharges with the Colorado River Simulation System and computed the profiles using hydraulic routing GENERATION OF THE and step-backwater methods (Bureau of ACCOUNTING SURFACE Reclamation, 1989a, b, 1990; V. LeGrand Neilson, Bureau of Reclamation, written The accounting surface was generated by commun., 1991). The discharges used to com using profiles of the Colorado River and annual pute the profiles of the various reaches of the high water-surface elevations of reservoirs, Colorado River for 1992-2001 are as follows: lakes, marshes, and drainage ditches. Where the Discharge, in cubic feet flood plain is wide and the river is near one side, Reach per second the geometry of the accounting surface was constructed from the combination of features Davis Dam to Parker Dam 15,511 that have the most influence on the water table Parker Dam to Headgate Rock Dam 12,570 near the edge of the flood plain. Static water- Headgate Rock Dam to Palo Verde Dam 10,890 level elevations in wells were not used to define Palo Verde Dam to Imperial Dam 9,646 the geometry of the accounting surface because static water levels in wells are affected by The water table in the river aquifer tributary inflow and most wells do not fully surrounding the reservoirs will be at or below the penetrate the river aquifer. The accounting maximum water-surface elevations of the surface was generated without consideration of reservoirs unless a significant source of local the time required for water to travel from the recharge is available to saturate the river aquifer river to any point of withdrawal from the river above that elevation. The accounting surface aquifer. The elevation and slope of the around reservoirs is flat and its elevation is accounting surface are shown on the maps by defined by the annual high water-surface contours that extend from the edge of the flood elevation used by the Bureau of Reclamation plain or shore of reservoirs to the river-aquifer to operate the reservoirs under normal flow boundary (fig. 3). The contours are oriented conditions (fig. 3A). The water-surface eleva approximately perpendicular to the general tion of Topock Marsh, 455 feet surveyed in direction of flow of the river and ground water 1992, was used to determine the elevation of the beneath the flood plain. The contours are curved accounting surface near the shoreline (pl. 10). and oriented to indicate interpreted water flow away from or toward the river or flood plain near Lakes Mead, Mohave, and Havasu bends in the river or places where the river enters or leaves areas underlain by the river aquifer The accounting surface around Lake Mohave (pl. 9). above Davis Dam was delineated using the annual high water-surface elevation of Water-surface profiles of the river were used 645 feet; the accounting surface around Lake to define the elevation and slope of the Havasu above Parker Dam is 449.6 feet accounting surface in much of the river valley (V. LeGrand Neilson, Bureau of Reclamation, below Davis Dam. Discharges used to compute written commun., 1991). The annual high water- the river profiles were determined on the basis of surface elevation varies at Lake Mead. The 26 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water water-surface elevation of Lake Mead since Flow continues in a southerly direction across 1940 ranged from 1,083.61 to 1,225.85 feet; the Cactus Plain to the east of Mesquite Mountain, maximum water-surface elevation in most years turns to the southwest, and returns to the flood is below 1,205 feet (Boner and others, 1991). plain between Mesquite Mountain and Moon Therefore, for the purpose of this report, the Mountain (pi. 15). The elevation and slope of accounting surface around Lake Mead was the accounting surface were set to river elevation delineated using the elevation of the fixed above Headgate Rock Dam, and water-surface spillway crest of Hoover Dam, which is elevations along the drainage ditches on the east 1,205.4 feet. side of the flood plain below Parker were adjusted by static water-level elevations in two Davls Dam to Topock wells southeast of Parker. From 4 miles above The accounting surface between Davis Dam Palo Verde Dam downstream to Draper Lake, the and Topock was generated from the river profile elevation of the accounting surface along the in the northern and western parts of the Mohave east edge of the flood plain was set to the river Valley (pi. 9). The water-surf ace elevation of profile (pi. 18). Topock Marsh was used to define the accounting Water was interpreted to flow approximately surface in the southeastern part of the valley. Flow directions in the river aquifer were parallel to the river along the northwest bank interpreted to be away from the river at the head below Parker and return to the flood plain along of the valley along the west bank and toward the the Riverside Mountains (pi. 12). Water flows river at Big Bend where the alluvial slope ends parallel to the flood plain between the Riverside against bedrock. Along the east side of the and Big Maria Mountains and along the valley, water was interpreted to flow away from Big Maria Mountains. The elevation of the the river to the southeast, move parallel to the accounting surface was set to the river profile flood plain toward the south, and return to from Headgate Rock Dam to 0.5 mile below Palo Topock Marsh and the river above the canyon at Verde Dam. The accounting surface beneath Topock (pi. 10). Water moves away from the Palo Verde Mesa from 3 miles below Palo Verde river for several miles below Big Bend along the Dam south to the north side of Smoketree Valley west side of the valley, moves parallel to the is defined by water-surf ace elevations in the flood plain, and returns to the river near Topock. drainage ditches near the west edge of the flood plain (pis. 15 and 18). The measurements of Parker Dam to Draper Lake water-surface elevation in the drainage ditches were made by Palo Verde Irrigation District Accounting-surface contours were drawn from the mouth of the canyon below Parker Dam during September and October 1989 when water 3.1 miles above Headgate Rock Dam (pi. 12) and levels were near the annual maximum elevation. along both sides of the river and flood plain The accounting surface in Chuckwalla Valley south to Draper Lake, which is about 12 miles is flat and was set to an elevation of 234 feet, south of Cibola, Arizona (pi. 16). The elevation which is the representative accounting-surface of the water surface in the river above Headgate elevation at the east end of Chuckwalla Valley Rock Dam is 364.4 feet (V. LeGrand Neilson, where it joins Palo Verde Mesa (pi. 15). The Bureau of Reclamation, written commun., accounting surface in Smoketree Valley is flat 1991). Water is interpreted to flow away from and was set to the representative elevation at the both sides of the pool above the dam to the intersection with the Colorado River flood plain northwest and to the south. of 212 feet (pi. 18). River profiles were used to Water was interpreted to flow to the south define the accounting surface from the north side beneath and to the east of Parker, and some water of Smoketree Valley south to Draper Lake returns to the flood plain southwest of Parker. (pi. 18). Generation of the Accounting Surface 27 Clear Lake to Laguna Dam the elevation of the accounting surface. The lowering of static water levels below the The accounting surface was set to the river accounting-surface elevation in wells in these profile from Clear Lake to 1 mile below areas would result in a change in the designation Martinez Lake (pis. 18 and 19). In the trough of the wells from yielding water that will be east of the Chocolate Mountains (also known replaced by tributary water to yielding water that locally as Laguna Mountains), water was will be replaced by Colorado River water. interpreted to flow east from the vicinity of Martinez Lake at an elevation of 185 feet, then Periodic monitoring and evaluation of southeast between the Chocolate and Middle channel conditions, river discharges, water- Mountains, and return to the river through the surface elevations in drainage ditches, and gap below Yuma Proving Ground at an elevation reservoir operations would provide information of 150 feet. Water-surface elevations above needed to determine if an adjustment to the Laguna Dam vary from 146 to 150 feet. The elevations of the accounting surface were accounting-surface elevation between Laguna warranted. Subsurface conditions in the river Dam and Imperial Dam was set to 150 feet. aquifer are poorly known near the boundaries of the river aquifer in many areas. Monitoring geologic and geophysical studies and well POTENTIAL ADJUSTMENTS TO THE drilling will provide new information that could METHOD allow refinement of the position of the boundaries of the river aquifer. The accounting surface was generated on the basis of profiles of the lower Colorado River computed for the highest median monthly SUMMARY projected discharge for 1992 2001, water- surface elevations in drainage ditches that Accounting for the use of Colorado River represent regulated flow conditions of the late water is required by the U.S. Supreme Court 1980's and early 1990's, delivery of full decree, 1964, Arizona v, California. Water allocations of river water to users in the United pumped from wells on the flood plain and from States, and river-channel conditions surveyed certain wells on alluvial slopes outside the flood between 1980 and 1988. Major changes in any plain is presumed to be river water and is of these conditions could result in water-surf ace- accounted for as Colorado River water. A elevation changes in the river channel and method was developed to identify wells outside drainage ditches, which could lead to an the flood plain of the lower Colorado River that adjustment of the accounting surface. yield water that will be replaced by water from Accounting-surface elevations in the river the river. The method provides a uniform aquifer around Lakes Mohave and Havasu are criterion of identification that is based on based on water-surface elevations required to hydrologic principles for all users that pump operate the reservoirs under normal flow water from wells. conditions. A change to either high-discharge or The method is based on the concept of a river drought-flow conditions could necessitate aquifer and an accounting surface within the changes in water-surface elevations required to river aquifer. The river aquifer consists of operate the reservoirs, which could lead to an permeable, partly saturated sediments and adjustment in the elevation of the accounting sedimentary rocks that are hydraulically surface. connected to the Colorado River so that water Future increases in pumping from existing can move between the river and the aquifer in wells or development of new well fields in areas response to withdrawal of water from the aquifer outside the flood plain could cause static water- or differences in water-level elevations between level elevations in wells that initially were above the river and the aquifer. The subsurface limit of the accounting surface to decline to or below the river aquifer is the nearly impermeable 28 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water bedrock of the bottom and sides of the basins highest median monthly projected discharges that underlie the Colorado River valley and in the Colorado River for 1992-2001 that adjacent tributary valleys. The accounting were determined by the Colorado River surface represents the elevation and slope of the Simulation System. The accounting surface unconfined static water table in the river aquifer around reservoirs is flat and its elevation is outside the flood plain and the reservoirs of the defined by the annual high water-surface Colorado River that would exist if the river were elevation used to operate the reservoir under the only source of water to the river aquifer. The normal flow conditions. The elevation of the accounting surface extends outward from the accounting surface is 645 feet around Lake edge of the flood plain or a reservoir to the Mohave and 449.6 feet around Lake Havasu. subsurface boundary of the river aquifer. This The elevation of the accounting surface around method provides a way to identify those wells Lake Mead is 1,205.4 feet, which is the elevation presumed to yield water that will be replaced by of the fixed spillway crest of Hoover Dam. water from the river by determining if the elevation of the static water table at a well is above or below the accounting surface. SELECTED REFERENCES Wells that have a static water-level elevation equal to or below the accounting surface are Anderson, R.E., 1973, Large-magnitude late Tertiary presumed to yield water that will be replaced by strike-slip faulting north of Lake Mead, Nevada: water from the river. Pumping water from a well U.S. Geological Survey Professional Paper 794, completed in the river aquifer where the 18 p. elevation of the static water level in the well 1978, Geologic map of the Black Canyon is below the elevation of the accounting surface 15-minute Quadrangle, Mohave County, will eventually cause movement of water Arizona, and Clark County, Nevada: U.S. from the river into the river aquifer. Wells that Geological Survey Geologic Quadrangle Map have a static water-level elevation above the GQ-1394, scale 1:62,500. accounting surface are presumed to yield water -1977, Geologic map of the Boulder City 15-minute Quadrangle, Clark County, Nevada: that will be replaced by water from precipitation U.S. Geological Survey Geologic Quadrangle and inflow from tributary valleys. If more water Map GQ-1395, scale 1:62,500. is pumped from the well than can be replaced Bagby, W.C., Haxel, G.B., Smith, D.B., Koch, R.D., from a tributary source, static water-level Grubensky, M.J., Sherrod, D.R., and Pickthorn, elevations in the well will decline below the L.G., 1987, Mineral resources assessment of the accounting surface and water will eventually Kofa National Wildlife Refuge, Arizona: U.S. move toward the well from the river. Geological Survey Open-File Report 87-609, The flood plain and adjacent alluvial slopes 82 p. in the lower Colorado River valley and in the Bales, J.T., and Laney, R.L., 1992, Geohydrologic tributary valleys are underlain by the river reconnaissance of Lake Mead National aquifer. The river aquifer includes the younger Recreation Area Virgin River, Nevada, to alluvium, the older alluviums including the Grand Wash Cliffs, Arizona: U.S. Geological Chemehuevi Formation, the Bouse Formation, Survey Water-Resources Investigations Report the fanglomerate, and the Muddy Creek 91-4185, 29 p. Formation, which overlies nearly impermeable Barrows, L.J., and Fett, J.D., 1991, A sloping wedge technique for calculating gravity terrain bedrock. The total thickness of the river aquifer corrections: Geophysics, v. 56, no. 7, ranges from 0 to more than 5,000 feet. p. 1061-1063. The accounting surface was generated by Bates, R.L., and Jackson, J.A., eds., 1987, Glossary using profiles of the Colorado River and water- of geology, 3rd ed.: Alexandria, Virginia, surface elevations of reservoirs, lakes, marshes, American Geological Institute, 788 p. and drainage ditches. The river profiles were Bell, J.W., and Smith, E.I., 1980, Geologic map of computed by the Bureau of Reclamation for the the Henderson Quadrangle, Nevada: Reno, Selected References 29 University of Nevada, Nevada Bureau of Mines and associated basins, southeastern Nevada and and Geology Map 67, scale 1:24,000. northwestern Arizona: Geological Society of Bentley, C.B., 1979a, Geohydrologic reconnaissance America Bulletin, v. 105, p. 501-520. of Lake Mead National Recreation Area Mount Boner, F.C., Konieczki, A.D., and Davis, R.G., 1991, Da vis to Davis Dam, Arizona: U.S. Geological Water resources data for Arizona, water year Survey Open-File Report 79-691, 34 p. 1990: U.S. Geological Survey Water-Data 1979b, Geohydrologic reconnaissance of Report AZ-90-1, 381 p. 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R.H., Jr., 1993, Seismic stratigraphy and Carr, W.J., 1991, A contribution to the structural tectonic development of Virgin River depression history of the Vidal-Parker region, California 30 Method to Identify Wells That Yield Water That Will be Replaced by Colorado River Water and Arizona: U.S. Geological Survey Nevada, Nevada Bureau of Mines and Geology Professional Paper 1430, 40 p., 5 plates. Open File 89-1, 154 p. Carr, W.J., and Dickey, D.D., 1980, Geologic map Evans, J.G., Sherrod, D.R., Hill, R.H., Jachens, R.C., of the Vidal, California, and Parker, and McDonnell, J.R., Jr., 1990, Mineral SW, California-Arizona Quadrangles: U.S. resources of the Mohave Wash Wilderness Geological Survey Miscellaneous Investigations Study Area, Mohave County, Arizona: U.S. Map 1-1125, scale 1:24,000. Geological Survey Bulletin 1704-A, 17 p. 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