SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS,

REGIONAL AQUIFER-SYSTEM ANALYSIS,

SOUTH-CENTRAL ARIZONA AND

PARTS OF ADJACENT STATES

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U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1406-A
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Summary of the Southwest Alluvial Basins, Regional 

Aquifer-System Analysis, South-Central Arizona and 

Parts of Adjacent States

By T.W. ANDERSON

REGIONAL AQUIFER-SYSTEM ANALYSISSOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1406-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement

by the U.S. Government

Library of Congress Catologing-in-Publication Data

Anderson, T.W. (Thomas Warren), 1940-

Summary of the Southwest Alluvial basins, regional aquifer-system analysis, south-central Arizona and parts of 

adjacent states / by "T.W. Anderson.

p. (Regional aquifer-system analysis southwest Alluvial basins, Arizona and adjacent states) (U.S. Geological 

Survey professional paper ; 1406-A) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19. 16:1406-A

1. Aquifers Arizona. 2. Groundwater flow Arizona. 3. Water chemistry. I. Series. II. Series: U.S. Geological 

Survey professional paper ; 1406-A. 

GB1199.3A7A55 1994

553.7'9'09791 dc20 94-11389

CIP

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Gordon P. Eaton 

Director

CONTENTS

Page

Introduction 2

Description of the study area 2 

Southwest Alluvial Basins,

Regional Aquifer-System Analysis 2

Geohydrology 4

Physical setting 4

Geologic setting 4

Geohydrologic units 5

Geophysical studies 6

Hydrologic properties 7

Categorization of basins 9

Regional ground-water flow system 12

Occurrence and movement of ground water 14

Regional ground-water flow system Continued 

Predevelopment flow system - 

Development flow system -

Geochemistry - 

Dissolved solids and other characteristics - 

Ground-water chemistry types - 

Trace elements - 

Geochemical models -

Water-resources development - 

History of development 

Effects of development 

Considerations for future development -

Selected references -

A14 

18 

20

20

21

21

21

22

22

23

28

30

ILLUSTRATIONS

FIGURE l.

2.

3,4.

5-7.

8.

9.

10.

11.

12, 13.

14.

15.

Map showing Basin and Range physiographic province and the Southwest Alluvial Basins study area 

Sketch showing generalized section and description of principal hydrogeologic units in basins of the study area 

Graphs showing:

3. Approximate range in hydraulic conductivity for the three alluvial facies differentiated by relative grain

S1Z6 - - . _. . ._.._.._.__... , ....... ........ ........ ......... .. .......... ........

4. Approximate range in specific yield for the three alluvial facies differentiated by relative grain size 

Maps showing:

5. Grouping of basins based on geohydrologic factors - 

Page

A3 

6

_________________.>__________. 10

6. Basins for which ground-water flow and geochemical models were developed 11

7. Predevelopment ground-water flow conditions and pattern of interbasin flow 

Sketch showing typical predevelopment inflow and outflow components in the alluvial basins 

Graphs showing relation of estimated mountain-front recharge in a basin to total precipitation on the basin and

total precipitation greater than 8 inches --------- -- ----------- -.---------- 

Graph showing annual ground-water pumpage, 1915-80 --------- ------------ --------- -------- ------

Diagram showing volume of water stored in the basin-fill aquifers to a depth of 1,200 feet below land surface, total

volume pumped in 1915-80, and estimated net ground-water depletion 

Maps showing:

12. Areas of water-level decline from the beginning of development through 1980 

13. Ground-water flow system in Stanfield basin, 1923 and 1976-77 - 

Photograph showing earth-fissure system more than 8 miles long, near Eloy, Arizona - 

Graphs showing generalized relation between increasing pumpage and changes in water-budget components 

13

15

17

19

19

24

25

27

29

TABLE

TABLE 1. Estimates of water budget before development 

Page

A15
VI CONTENTS

CONVERSION FACTORS AND VERTICAL DATUM

Multiply inch-pound units By To obtain metric units

inch (in.) 25.40 millimeter (mm)

foot (ft) 0.3048 meter (m)

square foot (ft2) 0.0929 square meter (m2)

foot squared per day (ft /d) 0.0929 meter squared per day (m /d)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

acre 0.4047 square hectometer (hm )

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

cubic mile (mi3) 4.168 cubic kilometer (km3 )

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft^/s) 0.02832 cubic meter per second (m3/s)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

In this report, temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) by 

the following equation:

°F = 1.8 x (°C + 32)

Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets 

of both the United States and Canada, formerly called "Sea Level Datum of 1929."
REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, 

REGIONAL AQUIFER-SYSTEM ANALYSIS, SOUTH-CENTRAL ARIZONA

AND PARTS OF ADJACENT STATES

By T.W. ANDERSON

ABSTRACT

In 1979, the U.S. Geological Survey began the Southwest Alluvial 

Basins, Regional Aquifer-System Analysis study to evaluate the availability of water resources, functioning of the alluvial-aquifer systems, 

aquifer properties, water-quality and geochemical characteristics, 

and the interrelation of the individual ground-water basins in a 

regional hydrologic system. Results of this study were published in 

a series of reports that described (1) the geohydrology of the alluvial 

basins, (2) the geochemistry of ground water, (3) the simulation of 

ground-water systems in the alluvial basins, and (4) a summary of 

the information presented in the other three reports (this report).

The 82,000-square-mile Southwest Alluvial Basins, Regional 

Aquifer-System Analysis study area contains 72 basins. Each basin 

was grouped into one of five broad categories on the basis of geohydrologic character of the aquifer material and total annual downvalley 

flow through the basin. The interrelation of these factors and the 

response of the systems to pumping stress are similar among basins 

of a particular group. The groups are labeled southeast, central, west, 

Colorado River, and highland.

Unconsolidated alluvial deposits form the principal aquifers of the 

area. In contrast, the surrounding bedrock of the mountains can be 

considered impermeable. The basins are filled with alluvial deposits 

that range from a few thousand feet to more than 10,000 feet in 

thickness. In almost all basins, the general vertical sequence of 

sedimentary units includes, in ascending order, pre-Basin and Range 

rocks, lower and upper basin fill, and stream alluvium. Each unit 

has different physical, geologic, and hydrologic properties largely 

because of differences in the depositional environment and source 

area of the sedimentary material. In places, the alluvial deposits 

include a range of grain sizes from clay to gravel and can be unconsolidated to well consolidated within a basin. The alluvial deposits 

are hydraulically interconnected and form a single aquifer system 

within each basin.

The alluvial-basin aquifers serve principally as reservoirs for the 

storage of water. An estimated 900 million acre-feet of recoverable 

ground water was stored in the upper 1,200 feet of alluvial deposits 

prior to development. Outflow from one basin as surface flow, groundwater underflow, or a combination of both serves as inflow to the next 

basin downgradient. The quantity of inflow entering and outflow 

leaving the basins of the study area annually is estimated to have 

been about 2.5 million acre-feet prior to development. The basins are

aligned in a dendritic pattern matching the surface-water drainage. 

Large-scale development in one basin could affect inflow to downgradient basins and outflow from upgradient basins.

The chemical quality of ground water generally is suitable for most 

uses in much of the area. Dissolved-solids concentrations range from 

about 300 to 1,000 milligrams per liter in most basins; locally, dissolved-solids concentrations are from 1,000 to more than 10,000 

milligrams per liter. Dissolution reactions and, in a few places, evapotranspiration are principal causes of dissolved-solids concentrations 

in excess of about 1,000 milligrams per liter. In small parts of several 

basins, trace elements are present in ground water in concentrations 

that exceed standards for public supply and certain agricultural uses.

Use of water resources in the area has resulted in depletion of 

surface flow in several basins and overdraft of ground water in most 

developed basins. In places, water levels have declined more than 400 

feet because of withdrawals in excess of natural recharge. Water-level 

declines of 50 to 200 feet are common in developed basins. From the 

time development began through 1980, an estimated 184 million acrefeet of ground water has been withdrawn. Nearly 50 percent of this 

historical pumpage represents net depletion of ground-water storage. 

The other 50 percent of this pumpage was supplied by a reduction of 

streamflow and evapotranspiration or increases in recharge as a 

result of increased infiltration of streamflow and return of excess 

irrigation water. The regional flow system was altered as a result of 

development. In 1980, discharge exceeded recharge in most developed 

basins. Ground-water flow rates and directions have been altered and 

the flow direction has been reversed in parts of a few basins. The result 

has been capture of part or all of the former natural discharge. Near 

Tucson, selected reaches of formerly perennial streams now flow only 

in response to precipitation because of extensive ground-water development and associated lowering of the water table. The change in the 

basin flow system depends on the geohydrologic setting, which is one 

factor incorporated into a system of basin categorization for this study. 

The changes associated with water-resources development, therefore, 

are predictable and are related to basin category.

Associated with the water-level declines and changes in basin 

flow components are ancillary problems of decreasing aquifer productivity with depth, variable water quality with depth, and 

increased incidence of land subsidence and earth fissuring. Land 

subsidence and the occurrence of earth fissures have been

Al
A2 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

documented in highly developed parts of central Arizona and are 

related to ground-water withdrawal and associated water-level declines. Earth fissures have occurred since about 1927. A fissure 

system near Eloy, Arizona, is more than 8 miles long and has had 

vertical offset of about 3 feet.

Results of ground-water flow modeling indicate that the effect of 

development on the ground-water systems of the individual basins 

and the changes' in magnitude of individual flow components generally are predictable for each group of basins. Sensitivity analyses 

of ground-water flow models indicate that a relation exists between 

basin category and sensitivity of the models to selected properties. 

The properties are affected by the degree of development in the 

basin, the geologic setting within the basin, and the magnitude of 

predevelopment downvalley flow. Information transfer among basins of a particular group proved to be applicable and facilitated 

the development and calibration of ground-water flow models. 

Geochemical models also provided a means of verifying existing 

conceptual models of ground-water flow systems and valuable insight into the significance and magnitude of components of the 

basin water budgets.

INTRODUCTION

Alluvial basins in south-central Arizona and parts 

of adjacent States contain aquifers that store and yield 

large quantities of ground water. In many basins, the 

aquifers have been the principal source of water to 

support economic development, which has been based 

predominantly on agriculture. In 1979, the U.S. Geological Survey (USGS) began a study of the southwest 

alluvial basins, as part of its nationwide Regional 

Aquifer-System Analysis (RASA) Program, to evaluate 

the status of knowledge available to define the extent 

and character of the aquifers, to document the effects 

of development on the water resources, and to explore 

similarities and differences among the many basinaquifer systems of the region.

DESCRIPTION OF THE STUDY AREA

The study area includes about 82,000 mi2 in arid to 

semiarid southern and central Arizona and parts of 

California, Nevada, New Mexico, and Mexico (fig. 1). 

The study area includes the surface-water drainage of 

the Colorado River between Hoover Dam and the international boundary near Yuma. The San Bernardino 

and Douglas basins in southeastern Arizona and parts 

of several basins in southwestern Arizona also are included, although these basins drain to Mexico. Also 

included are the Willcox basin in the southeastern part 

of Arizona and Hualapai Valley in the northwestern 

part (fig. 1), both of which have internal surface-water 

drainages. The area contains 72 basins that are virtually independent hydrologic entities that share common 

geologic and hydrologic characteristics. The boundaries 

between basins correspond to surface-water drainage

divides, ground-water divides, areas of minimal interbasin connection, and, in a few instances, sociopolitical 

boundaries.

The area is characterized by wide, gently sloping 

plains whose continuity is broken by sharply rising 

mountains. The plains are the surface of the alluvial 

deposits that fill the basins. The alluvial deposits 

range from a few thousand to more than 10,000 ft 

thick, include grain sizes from clay to gravel, and can 

be unconsolidated to well consolidated within a basin. 

The alluvial deposits are hydraulically interconnected 

and form a single aquifer system in each basin. The 

sediments that fill a basin are considered an aquifer 

system in accordance with the definition of Poland and 

others (1972). The basin-fill sediments probably include several permeable beds separated by finegrained confining units. In general, all the saturated 

sediments were hydraulically connected prior to development. As a result of large-scale withdrawals, however, variable degrees of hydraulic separation between 

permeable beds are indicated in some basins on the 

basis of differences in hydraulic head. Large quantities 

of ground water are stored in these aquifer systems. 

The occurrence and movement of ground water are 

influenced by the geology, physiography, and climate in 

the individual basins, which differ widely throughout 

the area.

SOUTHWEST ALLUVIAL BASINS, 

REGIONAL AQUIFER-SYSTEM ANALYSIS

The primary purpose of the Southwest Alluvial Basins, Regional Aquifer-System Analysis (Swab/RASA) 

study was to develop a better general understanding 

of the extent and workings of the hydrologic systems 

of alluvial basins in south-central Arizona and parts 

of adjacent States (Anderson, 1980). Goals included 

development of quantitative descriptions of the (1) 

physical framework of the aquifers, (2) predevelopment ground-water conditions, and (3) aquifer-system 

responses to development as related to the controlling geologic and hydrologic conditions. Numerical 

and geochemical models were the principal tools used 

in analyzing the geohydrologic systems; the models 

also provide predictive capabilities to explore and 

evaluate water-resources development options.

An initial hypothesis of the project was that certain 

characteristics and relations are common to many basins or subsets of basins. The strategy was to look in 

detail at selected basins that have an extensive data 

base and that typify a subset of basins having similar 

hydrologic characteristics. Information from basins in 

which the geohydrologic systems are well documented
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A3

110° 109° 108°

WEST 

SALT 

RIVER 

VALLEY

Phoenix

50

UNITED STATES

BASIN AND RANGE

PHYSIOGRAPHIC

PROVINCE

50

100 MILES

_J

L_

SALTON 

TROUGH

STUDY AREA SONORA 

DESERT

100 KILOMETERS

EXPLANATION

APPROXIMATE AREA OF 

"GILA LOW" (PEIRCE, 

1974)

BASIN BOUNDARY

BOUNDARY OF STUDY 

AREA

INDEX MAP 

FIGURE 1. Basin and Range physiographic province and the Southwest Alluvial Basins study area.
A4 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

was transferred to hydrologically similar basins for 

which only sparse data are available and evaluated 

using numerical or geochemical modeling. This approach served as an aid to describe and analyze the 

geohydrologic systems of all basins. For the most part, 

the study used existing data, which were accumulated 

from various State, Federal, and local water-resources 

agencies. Existing data were supplemented through the 

collection of additional water- and rock-chemistry data 

and through field application and testing of geophysical-exploration techniques. Lithofacies maps were compiled for many basins to evaluate geologic and lithologic 

similarities.

The results of the Southwest Alluvial Basins, RASA 

study are summarized in this report. Chapters B, C, 

and D in Professional Paper 1406 describe detailed 

results of the study. Chapter B (Anderson and others, 

1992) discusses the geohydrology of the alluvial basins 

and presents the general system of basin categorization, estimates of predevelopment water-budget components of each basin, and a history of ground-water 

development and associated effects. Chapter C (Robertson, 1991) describes the water quality and geochemistry of the aquifer systems. Chapter D (Anderson and 

Freethey, in press) discusses the numerical models developed and used to simulate ground-water flow in 

selected specific basins and in generalized basins representing average physical and hydrologic conditions 

for several categories of basins.

In addition to the Professional Paper series, about 

50 reports were produced as a result of this study. 

All reports produced by the project staff, contractors, 

and cooperators are listed in the selected references 

of this report.

GEOHYDROLOGY

The Swab/RASA study area is characterized by a 

series of alternating horst and graben features that 

make up the mountain ranges and basins that typify 

the Basin and Range physiographic province. The 

mountain ranges consist of nearly straight segments 

of consolidated rocks that are more or less parallel, 

predominantly trending in a north to northwest direction. The mountain ranges separate the area into 

a series of sediment-filled basins, which are the subject of this study.

PHYSICAL SETTING

The study area includes extensive parts of the Sonoran Desert and Mexican Highlands sections of the

Basin and Range physiographic province as defined 

by Fenneman (1931; see fig. 1, this report). The basins are structural depressions that were subsequently filled with alluvial material mostly eroded from 

the surrounding mountains. Each basin is characterized by a gently sloping alluvial plain enclosed or 

partly enclosed by mountain ranges. Basins of the 

Sonoran Desert section generally are oval shaped 

and consist of about 70 percent basin floor and 30 

percent mountains. Basins of the Mexican Highlands 

section tend to be longer and narrower than those of 

the Sonoran Desert section and consist of about 50 

percent basin floor and 50 percent mountains. Landsurface altitude of the basin floors increases from 

less than 500 ft above sea level in the southwest to 

more than 4,000 ft in the northeast. Altitudes of the 

mountains range from about 1,000 ft to more than 

10,000 ft above sea level. Precipitation, which increases in direct proportion to land-surface altitude, 

ranges from about 3 in./yr near Yum a in the southwest to more than 30 in./yr in the headwaters of the 

Gila, Salt, and Verde Rivers in the northeast (fig. 1). 

Average annual evaporation and length of growing 

season increase from the northeast toward the southwest as a function of the trend toward a hotter and 

drier climate in the lower altitude, more arid parts of 

the area.

As part of the process of evaluating basin similarities and developing basin categories, many physical 

and hydrologic characteristics of individual basins 

were measured. For all 72 basins, such characteristics as length, width, total area, area of alluvial material, average land-surface slope of the alluvial 

material, average annual precipitation on the basin, 

average predevelopment slope of the water table, and 

many others, were measured or calculated. Many derivative characteristics were subsequently calculated, 

such as length-width ratios and land-surface and 

water-table slope ratios. These data were tested 

using a statistical-analysis program to determine if 

correlations existed between various properties. In 

general, this process proved unsuccessful; it did not 

result in significant positive findings and did not facilitate the categorization process. The only statistically significant correlation that resulted was 

between average annual precipitation and altitude of 

land surface.

GEOLOGIC SETTING

The mountains of the study area consist of igneous, 

metamorphic, and sedimentary rocks that range in 

age from Precambrian to Cenozoic (Dane and Bach-
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A5

man, 1965; Wilson and others, 1969). The rocks are 

chiefly granite, gneiss, schist, and volcanic types. 

Sedimentary rocks make up a small percentage of 

the mountains in the southeastern part of the study 

area and near the north boundary. The igneous, 

metamorphic, and sedimentary rocks that make up 

the mountains also form the lateral and underlying 

boundaries of the alluvial-basin aquifers.

The basins were formed during middle to late Miocene time Basin and Range structural disturbance. 

The structural disturbance occurred between 15 and 

10 m.y. (million years) ago in the western part of the 

area (Shafiqullah and others, 1980) and between 12 

and 10 m.y. ago in the eastern part (Scarborough and 

Peirce, 1978). Movement along high-angle normal 

faults downdropped basins in relation to the mountain masses and resulted in a series of generally 

north- to northwest-trending basins. Basin subsidence 

was a gradual process that was accompanied by deposition of locally derived sediment in internally drained 

basins. This closed-drainage environment resulted in 

the gradational pattern of coarse-grained sediment 

near the mountains and fine-grained sediment near 

the basin center exhibited in the basin-fill units. The 

depositional centers of the basins typically contain 

evaporite deposits, such as gypsum, anhydrite, and 

halite, in addition to fine-grained sediment.

On the basis of two-dimensional gravity modeling 

results (Oppenheimer and Sumner, 1980), the deepest basins are those in the central part of Arizona. 

Depth to underlying bedrock may exceed 10,000 ft in 

most of these basins, which are part of the "Gila 

Low" defined by Peirce (1974) (fig. 1). This area is 

characterized by the presence of massive evaporite 

deposits in the basin fill and has been a center of 

deposition since the time of the Basin and Range 

structural disturbance. The area is still the focal 

point of a large part of the Gila River drainage. 

Major tributaries to the Gila River the Santa Cruz, 

Agua Fria, and Hassayampa Rivers and the Salt 

River and its principal tributary, the Verde River 

(fig. 1) all join the Gila River near Phoenix in the 

central part of the Gila Low. The drainage above this 

point includes 86 percent of the total Gila River 

drainage and 60 percent of the entire study area.

GEOHYDROLOGIC UNITS

The bedrock that underlies the basins and constitutes the mountains contains usable amounts of 

ground water only where extensively fractured or 

faulted. Such secondary permeability features may 

provide limited amounts of water, generally sufficient

only for domestic and stock supplies. A regional 

ground-water flow system involving hydraulic interconnection through the bedrock does not exist in the 

bedrock as it does in the Great Basin section of the 

Basin and Range province in Nevada (Burbey and 

Prudic, 1991).

Basin subsidence and sediment deposition occurred 

at differing rates throughout the area. As a result, 

the thickness, areal extent, and grain size of the alluvial deposits are highly variable from basin to basin. 

The alluvial deposits are made up of clay- to gravelsized material that may be unconsolidated to well 

consolidated in a single basin. The general vertical 

sequence of sedimentary units that overlie the bedrock consists of (1) pre-Basin and Range rocks, (2) 

lower and upper basin fill, and (3) stream alluvium 

(fig. 2). The units are distinguishable on the basis of 

structural relation, degree of consolidation and deformation, source area of clasts, geologic age, and 

water-bearing characteristics, and they reflect different conditions and intervals of deposition. Ground 

water occurs in the interstitial voids of the alluvial 

deposits, and the deposits are hydraulically interconnected and form a single aquifer system in most basins. Although the geohydrologic setting of each basin 

is unique, general spatial patterns in the character, 

thickness, and extent of the units can be documented 

within groups of basins. This was a major factor 

upon which the basin categorization was based 

(Anderson and others, 1992).

The pre-Basin and Range rocks consist of moderately to well-consolidated continental sedimentary 

deposits that range from clay to gravel and include 

some volcanic rocks. The sediments were deposited 

prior to the disturbance that formed the general 

basin and range topography and are faulted and tilted similar to the underlying bedrock. The pre-Basin 

and Range rocks generally are deeply buried in the 

basins and are unconformably overlain by basin-fill 

sediment. Although a few wells penetrate the preBasin and Range rocks near the basin edges, little is 

known of their water-bearing character. Pre-Basin 

and Range rocks generally are well indurated and 

are not a productive water-bearing unit. Where highly fractured, however, the unit may yield large quantities of water to wells.

Basin-fill sediments, which constitute the principal 

aquifer system in most basins, were deposited in basins formed by the Basin and Range structural disturbance. The lower part of the basin fill is 

moderately to highly consolidated, has extensive 

basin-center fine-grained material, and typically contains evaporite deposits that range from disseminated gypsum to massive halite beds. Lower basin fill
A6 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

was deposited in topographically closed basins. 

Upper basin fill was deposited during a transition period in which the basin drainage evolved from closed 

to integrated. The upper basin fill is less consolidated, generally is thinner, and contains less finegrained sediment than the lower basin fill. Both 

lower and upper basin-fill units grade from coarsegrained sediment at the basin margins to finegrained sediment near the basin center.

Stream alluvium consists of unconsolidated clay- to 

boulder-size material that was deposited contemporaneously with the establishment of the present surfacedrainage system. Where saturated, stream alluvium 

is the most productive part of the aquifers.

The alluvial-aquifer system in most basins serves 

mainly as a reservoir for the storage of ground water. 

The volume of water in storage is large compared

BEDROCK 

PEDIMENT

with the small quantity of flow occurring from basins 

of higher altitude to basins of lower altitude.

GEOPHYSICAL STUDIES

Several geophysical methods were used to evaluate 

subsurface geohydrologic conditions in basins in 

southern and central Arizona as a part of the 

Swab/RASA project (Tucci, 1989). Surface-geophysical 

methods, including gravity, seismic, and resistivity, 

were used to provide regional and local subsurface 

information. Borehole methods were used to provide 

information on lateral and vertical variations of geohydrologic conditions. The geophysical methods generally were applied in areas where additional 

geologic data were available that could be used to

BEDROCK 

PEDIMENT

GENERAL AREAS

OF HETEROGENEOUS

COARSE-GRAINED SEDIMENT

GENERAL AREA GENERAL AREAS

OF FINE-GRAINED OF HETEROGENEOUS

SEDIMENT COARSE-GRAINED SEDIMENT

s , UPPER BASIN ' 

'l>''\- FILL~

POORLY TO WELL-DEFINED TRANSITION 

FROM LOWER TO UPPER BASIN FILL

Greater than 60 percent_fjriesjj^' ; _' ; 

;\v-^: 55-80 percent finesy.

Greater than . 

80 percent fines

1N LOWER

BASIN

FILL ,

CENTRAL BASIN GRABEN

Not to scale

EXPLANATION

STREAM ALLUVIUM

BASIN FILL Shows a facies change 

from coarse grained to fine grained

PRE-BASIN AND RANGE SEDIMENTARY 

AND VOLCANIC ROCKS

BEDROCK OF THE MOUNTAINS

NORMAL FAULT Arrows show 

relative vertical movement

FIGURE 2. Generalized section and description of principal hydrogeologic units in basins of the study area.
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A7

correlate with and to evaluate the results of the geophysical methods.

Four general geohydrologic features or targets were 

identified for investigation using one or several geophysical methods. These targets were (1) basin structure, (2) basin stratigraphy, (3) hydrologic properties 

of the aquifer, and (4) depth to the water table. Structural targets included depth to the bedrock surface 

that defines the boundaries of the main water-bearing 

units and the location of buried faults in both bedrock 

and sediments. Stratigraphic targets included the contact between the upper and lower basin-fill units and 

the location of extensive fine-grained deposits in the 

basin fill. Porosity of basin-fill deposits, calculated 

from density values, was the hydrologic property 

sought from the borehole-gravity study.

Gravity data were used to develop a generalized 

depth-to-bedrock map for most of the Swab/RASA 

study area (Oppenheimer and Sumner, 1981). The 

gravity method was a useful reconnaissance tool in 

providing regional information because data collection is rapid and relatively inexpensive. Although 

this method provides useful information on basin size

and shape, it cannot provide information on detailed 

subsurface stratigraphy.

Seismic refraction was used northwest of Tucson to 

locate the water table and to detect velocity variations 

in the sediment that may correspond to different 

basin-fill lithologic units. Bedrock was readily distinguished, and a buried fault could be inferred. Although 

expensive for large-scale regional investigations, this 

method can provide detailed subsurface information.

In Tucson basin and Avra Valley (fig. 1), electricalresistivity soundings were used to locate fine-grained 

deposits, which are characterized by resistivities of 

less than 10 ohm-meters (Tucci, 1984). Shallow bedrock also was readily detected, and faults were indicated by displacement of the bedrock. The method 

was not successful in locating the water table because of the heterogeneous nature of the basin fill. 

The method generally is less expensive than the seismic method and can provide detailed subsurface 

information.

Borehole-gravity data were obtained in six wells in 

four basins to determine the density of the basin fill. 

Density values calculated from borehole-gravity data

DESCRIPTION OF PRINCIPAL HYDROGEOLOGIC UNITS

Unit

Stream alluvium

Upper basin fill

Lower basin fill

Lithology

Unconsolidated alluvial deposits along major streams and washes; 

ranges from boulders and gravel to minor amounts of lacustrine 

clay; typically consists of well-sorted sandy gravel with some silt.

Unconsolidated to moderately consolidated alluvial deposits. 

Grades from coarse- to fine-grained sediments toward the center 

of most basins; includes basin-center facies with greater than 60 

percent fine-grained silt and clay; also includes lake deposits and 

basalt flows in a few basins.

Weakly to highly consolidated gravel, sand, silt, and clay and in 

places contains interbedded to massive evaporites and volcanic 

rocks.

Range in thickness, 

in feet

0 to more than 

300

Less than 100 to 

1,000

Range in hydraulic 

properties

Only locally saturated. 

Hydraulic conductivity, 

30-1,000 feet per day. 

Specific yield, 15-25 

percent.

Hydraulic conductivity, 

1-100 feet per day. 

Specific yield, 3-25 

percent.

Hydraulic conductivity, 

1 50 feet per day. 

Storage coefficient, 

10-5 to 10-1.

Weakly to highly consolidated fanglomerate and alluvial 0 to 1,000 

deposits found on the perimeter of most basins. In some 

places grades rapidly into the fine-grained facies near major 

basin faults; underlies the fine-grained facies in extensive 

areas of many basins.

Basin-center fine-grained facies, weakly to moderately 1,000 to 5,000 

consolidated silt and clay, generally 55 to 80 percent finegrained sediments; contains few evaporite deposits.

Basin-center fine-grained facies, moderately consolidated silt Less than 1,000 

and clay, greater than 80 percent fine-grained sediments; 

gypsiferous in many places. Includes extensively 

interbedded or massive evaporites and interbedded volcanics 

in a few basins.

Pre-Basin and Range 

sediments and 

volcanics

Moderately to highly consolidated fanglomerate and alluvial 

deposits and interbedded volcanics that are structurally 

disturbed. Range in composition from silt, clay, claystone, and 

limestone to gravel and conglomerate and include interbedded 

volcanics in places.___________________________

Less than 1,000 

to 15,000

Hydraulic conductivity, 

1-13 feet per day.

Storage coefficient, 

6xlO-5 to ID" 2 .

Bedrock of the 

mountains

Igneous, metamorphic, and sedimentary rocks.

FIGURE 2. Continued.
A8 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

are not significantly influenced by casing, borehole 

roughness, or drilling-induced formation damage. 

Basin-fill densities ranged from 1.73 to 2.46 grams 

per cubic centimeter (Tucci and others, 1983) and are 

similar to those obtained by other investigators in 

Arizona and Nevada (Healey, 1970; Eaton and others, 

1972; Mattick and others, 1973). Basin-fill density 

generally increases with increasing depth. Porosity of 

saturated basin fill calculated from the boreholegravity data range from 17 to 33 percent. Boreholegravity surveys are expensive but can be justified if 

detailed basin-fill density data are needed.

An integrated approach incorporating several geophysical methods can provide valuable geohydrologic 

information before an expensive and extensive program of exploratory drilling is initiated in a basin. 

Gravity data can be acquired to provide general information on basin shape and depth. Resistivity data 

can be used to aid in determining location, areal extent, and thickness of fine-grained deposits, particularly near the basin center. Seismic data can provide 

additional stratigraphic and water-table information. 

All three geophysical methods can provide structural 

information near the basin margins. Programs are 

readily available for computers and hand calculators 

to aid in the interpretation of the geophysical data.

HYDROLOGIC PROPERTIES

Ground-water movement and storage are controlled 

by the hydrologic properties of an aquifer or confining unit. The hydrologic properties are, in turn, related to the textural characteristics, particularly 

average grain size of the material. The alluvial sediments that fill the basins generally can be grouped 

into three facies based on their textural character 

(1) the coarse-grained facies, (2) the fine-grained facies, and (3) the intermediate facies. The vertical 

transition from one facies to another is distinct in 

some places and difficult to discern in other places. 

Laterally, the transition may be distinct and occur 

abruptly or may be a gradational change from coarse 

to fine over a distance of many miles. The coarsegrained facies corresponds to stream alluvium (fig. 2), 

which is associated with the major surface drainages. 

Stream alluvium typically consists of well-sorted 

sand and gravel but also includes material ranging 

in size from silt to boulders. The fine-grained facies 

occurs in the lower and upper basin-fill units and 

consists of 55 percent or more of material less than 

0.0625 millimeters in diameter. The intermediate facies also includes sediments of the lower and upper 

basin-fill units and is the part of the alluvial deposits

not included in the fine- or coarse-grained facies. The 

intermediate facies consists of a heterogeneous, poorly sorted mixture of gravel, sand, silt, and clay. These 

three major alluvial facies of significantly different 

physical characteristics represent a first approximation of the hydrologic properties of the aquifers that 

control the ground-water flow system within each 

basin.

Because of its coarse-grained nature, stream alluvium can transmit and store large quantities of water. 

Hydraulic conductivity commonly ranges from about 

30 to as much as 1,000 ft/d (fig. 3), and specific yield 

ranges from 15 to 25 percent (fig. 4). The stream alluvium was only locally saturated but was a significant factor in the predevelopment flow system. 

Although significant dewatering of the unit has occurred in response to development, it remains an 

important part of the aquifer system through its 

functional role of accepting and storing recharge from 

intermittent surface runoff.

The fine-grained facies has a large porosity and 

stores large quantities of water; however, wells finished in this facies have small yields because the hydraulic conductivity is small and water is not readily 

released from storage. The hydraulic conductivity 

typically is less than 10 ft/d on the basis of available 

aquifer-test data (fig. 3); specific yield probably ranges from less than 1 to 10 percent (fig. 4). The fineFACIES AND RELATIVE

GRAIN SIZE __________________________

Stream alluvium coarse

Alluvial deposits intermediate

Alluvial deposits fine

0.1 1.0 10 100 1,000 

HYDRAULIC CONDUCTIVITY, IN FEET PER DAY

FIGURE 3. Approximate range in hydraulic conductivity for the 

three alluvial facies differentiated by relative grain size .

FACIES AND RELATIVE 

GRAIN SIZE

Stream alluvium coarse

Alluvial deposits intermediate

Alluvial deposits fine

0 5 10 15 20 

SPECIFIC YIELD, IN PERCENT

FIGURE 4. Approximate range in specific yield for the three alluvial facies differentiated by relative grain size.

25
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A9

grained fades in the lower and upper basin-fill units 

greatly influences the general movement of ground 

water. Because of the small vertical and horizontal 

hydraulic conductivity, a small quantity of water 

moves through the facies. The small rate at which 

water moves into and out of the fine-grained sediment limits well production and recharge rates. 

Perched-water zones have developed and overlie 

parts of the fine-grained facies and interbedded lenses of fine-grained sediment. Where distinct lateral 

changes in grain size occur from fine to coarse, 

pumped wells that penetrate the coarse-grained material may exhibit drawdown responses indicative of 

nearby no-flow boundaries. The distribution of the 

fine-grained facies influences ground-water quality; 

areas underlain by thick fine-grained facies also are 

prone to land subsidence, which accompanies drainage and inelastic compaction of the sediment.

Regional patterns of the occurrence of the basincenter fine-grained facies were documented as part of 

this study (Freethey and others, 1986). Three distinct 

types of fine-grained facies are present. Two types 

occur in the lower basin-fill unit and are (1) a silt 

and clay facies that consists of 80- to 100-percent 

fine-grained material, which includes gypsum, gypsiferous mudstone, and massive evaporite deposits, and 

(2) a moderately fine-grained facies that consists of 

55- to 80-percent silt and clay, which can include 

evaporite deposits. The third type occurs in the upper 

basin fill and consists of greater than 60-percent finegrained sediment in the basin centers; this type does 

not contain evaporite deposits.

The alluvial facies of intermediate grain «ize is the 

principal water-yielding facies of the basin-fill deposits. This facies includes a range from fine-grained to 

coarse-grained material and is much more heterogeneous than the other two facies. The hydraulic conductivity ranges from 1 to 300 ft/d (fig. 3), and 

specific yield ranges from 5 to 25 percent (fig. 4).

The water-bearing character of the deposits in the 

deep part of the alluvial basins is not known because 

development of water supplies from great depths has 

not occurred. Presumably, low porosity and permeability would physically limit ground-water development because of greater compaction and cementation 

of sediment at depths of several thousand feet. Information to define the water-bearing properties generally is not available for depths below about 2,000 ft.

CATEGORIZATION OF BASINS

The basins of the study area were categorized on 

the basis of similarities in geologic, hydrologic, and

geochemical characteristics. Categorization was a 

means of grouping all basins so that the study of a 

selected few would provide information that could be 

transferred to other basins of the group. Details of the 

procedures used for the geohydrologic categorization 

are presented by Anderson and others (1992) and for 

the geochemical categorization by Robertson (1991). 

The two principal factors used for categorization were 

(1) the geohydrologic properties of the aquifer material and (2) the total annual downvalley flow through 

the basin. These factors largely control the manner in 

which the aquifer system responds to development. 

The general response of an aquifer system can be predicted by recognizing similarities in the controlling 

geohydrologic factors and the validity of information 

transfer between basins of a particular category. On 

the basis of the model results for several specific basins in each category, information transfer was determined to be a valid approach to analyzing system 

responses, provided that no major differences in geology or hydrology exist between the basin being analyzed and those basins from which the typical 

characteristics of the category were defined.

The principal geologic property that controls the 

occurrence and movement of ground water is the 

grain size of the sediment that makes up the basinfill aquifer system. Fine-grained sediments have the 

greatest control on the geohydrology because of their 

small hydraulic conductivity. The most important hydrologic factor in comparing the basin-fill aquifer systems is the total annual downvalley flow, which 

represents the total renewable water resources available on an annual basis prior to development. On the 

basis of areal similarities in grain size and downvalley flow, the basins were grouped into five categories: 

(1) southeast, (2) central, (3) west, (4) Colorado River, 

and (5) highland (fig. 5). The response of a groundwater system to the stresses of development will be 

similar among the basins of a particular category.

The understanding of the functioning of the 

ground-water system water movement and direction 

and the response of the system to stresses in each 

type of basin was aided by the development and application of ground-water flow models. Two types of 

ground-water flow models were developed and used; 

these are (1) specific-basin models and (2) basingroup models of generalized hydrologic systems that 

represent average conditions for basins of a particular category. Models of specific basins of the southeast, central, west, and Colorado River groups were 

developed to simulate actual physical systems and 

the documented system responses to historical development. Because of the absence of data, no models 

were developed of basins of the highland group. Each
A10 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

r

070 5° 114° 113 C 112° UTAH 111° 110° 109° 10f

l' T "1 ARIZONA 1 r" i

Xs 1

| ,Q\ ,

v] ! ¥ i

Hoover 5/7 i ^^°^ > c ' 

Darn // K rOY^-^^ <£,^. \ !

36°

50

I

100 MILES

I 

50 100 KILOMETERS

EXPLANATION

ALLUVIAL DEPOSITS Locally may include 

evaporite deposits and volcanic rocks

BEDROCK OF THE MOUNTAINS 

BOUNDARY OF STUDY AREA 

BASIN BOUNDARY

SUMMARY CATEGORIES OF BASINS 

SOUTHEAST

CENTRAL 

COLORADO RIVER

WEST

HIGHLAND

FlGUEE 5. Grouping of basins based on geohydrologic factors.
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES All

specific-basin model was based on available data and 

geologic and hydrologic concepts developed from previous investigations. A total of 12 specific-basin models were developed and were history matched to a 

predevelopment or an appropriate steady-state development condition (fig. 6). Ten of the specific-basin

114" 113 112=UTAH 1H C 110 109^ 108°

50

I

100 MILES

50 100 KILOMETERS 

EXPLANATION

ALLUVIAL DEPOSITS Locally may include 

evaporite deposits and volcanic rocks

BEDROCK OF THE MOUNTAINS 

BOUNDARY OF STUDY AREA

GROUND-WATER FLOW MODEL OF 

BASIN

GEOCHEMICAL MODEL OF BASIN 

BASIN BOUNDARY

FIGURE 6. Basins for which ground-water flow and geochemical models were developed.
A12 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

models were further evaluated using available transient or development data.

The basin-group models were developed using average values of physical and hydrologic properties for 

groups of basins that have similar geohydrologic environments. The models were used to evaluate the significance of particular hydrologic properties or flow 

components on modeling results and to systematically 

explore sensitivity to variations in property values 

used in the models. Two basin-group models were developed, one representing average conditions for the 

southeast basin group and the other representing average conditions for the central and west basin groups.

All modeling was accomplished using the threedimensional finite-difference model described by 

Trescott (1975) and Trescott and Larson (1976). A 

quasi-three-dimensional approach was used in which 

the aquifer system is simulated as several layers. 

Each layer represented a vertical segment of the 

basin-fill aquifer connected hydraulically by a leaky 

confining layer, represented by a leakance coefficient, 

to allow vertical flow between layers. Information 

necessary for the models includes the areal extent 

and thickness of the aquifer, water-bearing properties 

of the aquifer such as horizontal and vertical hydraulic conductivity, specific storage, ranges and locations 

of ground-water recharge and discharge, and pumpage. Results from the models consist of simulated hydraulic heads at specified locations, depths, and 

times; a mass balance that represents the summation 

of flow into and out of the simulated system; and 

simulated change in storage.

The properties that are significant in development 

of ground-water flow models are related to (1) hydrologic boundaries, (2) flow components, and (3) values 

of hydrologic properties. Hydrologic boundaries represent the physical constraints on aquifer extent and 

include water-level configuration, aquifer bottom, and 

lateral boundaries. Flow components include recharge to and discharge from the aquifer and changes in storage in the aquifer. Hydrologic properties 

control the rate of movement and quantity of storage 

in the aquifer. The model represents a much generalized system that uses average property values for 

large segments of the aquifer. The high degree of 

spatial variability that exists in the actual system 

must be simplified for use in the model. Because of 

the differences in geohydrologic setting and because 

different properties may be more significant in each 

group of basins, numerical models have various levels of data requirements.

Basins of the southeast group for which groundwater flow models were developed are upper San 

Pedro, Benson, lower San Pedro, Willcox, and Douglas

basins (fig. 6). These basin aquifers were simulated as 

two layers with an intervening leaky confining bed. 

Properties of particular significance include specific 

yield, interlayer leakance, the relation between evapotranspiration and depth to water, and pumpage quantity and location.

Basins of the central group for which ground-water 

flow models were developed are Tucson basin and 

Avra Valley (fig. 6). Models of the west group of basins were developed for Waterman Wash basin, Harquahala Plain, and McMullen Valley (fig. 6). These 

aquifer systems are best simulated using multiple 

layers. Recharge and discharge mechanisms are more 

important in the central group than in the west 

group because of their much larger magnitude. Storage depletion is common in both groups as a result of 

development, although some capture of discharge occurs in the central basins during the early phase of 

development. Location and magnitude of pumpage 

and values of specific yield are critical properties for 

use in models of basins of these two groups. Reliability of these data will determine the overall reliability 

of the simulation.

Basins of the Colorado River group for which 

ground-water flow models were developed are Parker 

Valley and Mohave basin (fig. 6). Because the hydrologic system of these basins is dominated by flow in 

the Colorado River, flow and aquifer-system properties related to shallow water-table conditions are 

most important. These properties include riverbed 

leakance, relation between evapotranspiration and 

depth to the water table, and return of excess applied 

irrigation water. Realistic ground-water flow models 

of these basins can be developed using a two-dimensional approach. Because of insufficient data, specificbasin models were not developed for the highland 

group, although they are assumed to be scaled-down 

versions of the Colorado River basins.

REGIONAL GROUND-WATER FLOW SYSTEM

The Southwest Alluvial Basins, RASA study area is 

composed of a series of ground-water reservoirs that 

are variably interconnected in a dendritic pattern 

closely parallel to that of the regional surface-water 

drainage (fig. 7). The surface-drainage network consists of the Colorado River; its principal tributary, 

the Gila River; and streams tributary to either the 

Colorado or Gila Rivers. Part of the area drains 

southward into Mexico, and two basins have internal 

surface drainage. Hualapai Valley in the northwestern part of Arizona, for the most part, drains to Red 

Lake Playa; the northernmost part of the valley
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A13

37

115° 109° 108°

50

I

100 MILES

ALLUVIAL DEPOSITS Locally may include 

evaporite deposits and volcanic rocks

50 100 KILOMETERS

EXPLANATION

GENERAL DIRECTION OF GROUND-WATER FLOW 

PREDEVELOPMENT BASIN OUTFLOW/INFLOW 

Predominately or solely as surface flow 

Predominately or solely as ground-water flow

BEDROCK OF THE MOUNTAINS 

BOUNDARY OF STUDY AREA 

BASIN BOUNDARY

FIGURE 7. Predevelopment ground-water flow conditions and pattern of interbasin flow.
A14 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

drains directly to the Colorado River. Willcox basin in 

the southeast corner of Arizona drains to Willcox Playa. The infrequent surface flow that reaches the 

playa areas is discharged by evaporation. The 

ground-water system of Hualapai Valley is throughflowing and discharges into the Colorado River 

(Laney, 1979). The ground-water flow system of 

Willcox basin is internally drained, as is the surfacewater system. Prior to development, discharge occurred as evaporation and transpiration at Willcox 

Playa (Meinzer and Kelton, 1913).

The regional pattern of interbasin flow that existed 

prior to development is illustrated in figure 7. The 

degree of interconnection between basins and the 

quantity of flow from one basin into the next downgradient basin are highly diverse. Interbasin flow occurs as surface-water flow, ground-water flow, or a 

combination of surface-water and ground-water flow 

(fig. 7, section A-A'). Section A-A' extends from the 

upper Hassayampa basin southward to the Gila 

Bend basin. Three conditions, illustrated proceeding 

in the downstream direction, include discharge by a 

combination of surface-water and ground-water flow, 

by ground-water flow only, and by almost entirely 

surface flow. No general pattern, such as increased 

flow in the downstream direction from one basin to 

the next downgradient basin, was found. The major 

controls on the geohydrology of individual basins and 

groups of basins include the physiographic factors of 

altitude and climate and the geologic factors of structure and lithology of basin sediments. These factors 

influence the quantity of flow that occurs between basins and the mechanism(s) by which it occurs.

OCCURRENCE AND MOVEMENT OF GROUND WATER

Ground water generally occurs under unconfined conditions in the alluvial aquifers. Ground water is confined in some parts of several basins where extensive 

fine-grained deposits overlie the principal water-bearing unit. Locally confined conditions are exhibited 

where coarse- and fine-grained sediments interfinger. 

The extent of the areas exhibiting confined conditions 

depends on the lateral extent and continuity of the 

fine-grained confining bed. Significant variations in hydraulic head with depth indicate upward vertical gradients in some locations and downward vertical 

gradients in other locations (Halpenny and others, 

1952; Davidson, 1973).

Ground-water levels in the basin-fill deposits range 

from above land surface along perennial streams to 

more than 600 ft below land surface near the mountain fronts in some basins. In nearly all basins, depth

to the water table is shallowest near the location of 

discharge from the basin, which can range from a 

small area at the downstream end of the basin to essentially a line sink along the major drainage 

through the length of the basin. The slope of the 

water table typically is less than the slope of the 

land surface. This difference in slopes results in the 

greatest depths to ground water along the mountain 

fronts at the greatest distance from the major drainage in the basin. This depth generally occurs in the 

area from midway to three-quarters of the distance 

from the lower to the upper end of the basin, although the exact setting is dependent on the individual basin characteristics.

Water-level contours indicate the general direction 

of ground-water movement in a basin. The shape of 

the water-level contours also indicates general areas 

of inflow and outflow. In some basins, the water-level 

contours are U- or V-shaped, and in other basins, 

water-level contours are nearly straight lines perpendicular to the basin axis. Basins in which contours 

are U- or V-shaped and nearly parallel to the mountain fronts receive significant quantities of mountainfront recharge. Ground-water movement is from the 

basin boundary toward the basin axis, and significant discharge generally occurs along the line sink, 

which is the major surface drainage of the basin. Basins in which contours are relatively straight and 

nearly perpendicular to the mountain fronts receive 

little mountain-front recharge. Generally, data are 

not available to document the exact shape of waterlevel contours at the mountain fronts in these basins. 

Recharge in these areas is small and sporadic. The 

general shape of water-level contours in these basins 

may lead to the conclusion that ground-water flow 

may be through the entire basin from the upstream 

to the downstream ends with only small contributions from the lateral mountain-front areas. Results 

of model analyses are insensitive to the magnitude 

and spatial distribution of recharge used in basin 

simulations where recharge is small. The rate of 

ground-water movement in the alluvial deposits depends on the physical and hydrologic characteristics 

of the material and on the hydraulic gradient. 

Ground-water movement generally ranges from a few 

feet to a few hundred feet per year on the basis of 

information on hydraulic gradient, porosity, hydraulic 

conductivity, and transmissivity.

PREDEVELOPMENT FLOW SYSTEM

Prior to development of the ground-water and surface-water resources, the regional ground-water flow
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A15

system and the individual basin systems were assumed 

to be in equilibrium long-term recharge was equal to 

long-term discharge and no change in storage occurred. 

The basis for the assumption that equilibrium conditions existed lies in the absence of major humanmade 

impacts on the ground-water systems. Natural variations probably were occurring as long-term trends or 

cyclic variations over decades and centuries. These natural variations were assumed to be small compared 

with the impacts of humans beginning in the early part 

of the 20th century.

A conceptual model of the predevelopment hydrologic 

system of the entire area was developed as part of this 

study (Freethey and Anderson, 1986). The quantities 

of recharge and discharge and the volume of water in 

storage were estimated from field data, numerical 

modeling, and information transfer. The transfer of 

information was based on known similarities in physiographic setting and in lithology of the basin-fill sediments between adjacent or nearby basins.

Ground-water inflow to the basin-fill aquifers occurs 

primarily through three mechanisms: (1) infiltration of 

runoff along the major streams, (2) infiltration along 

the mountain fronts, and (3) underflow from adjacent 

basins (fig. 8). The total average annual inflow to all 

the basins of the study area for predevelopment conditions is estimated to have been 2.5 million acre-ft. In 

general, water enters an aquifer at the upstream end 

of a basin and along the mountain fronts, flows toward 

the basin center, and exits at the downstream end. The 

specific flow characteristics in a basin or for a group of 

basins are dependent on the hydrologic properties of 

the aquifer material and on the total annual flow 

through the aquifer.

The estimated quantities of inflow to the study 

area before development are summarized in table 1.

TABLE 1. Estimates of water budget before development, in acrefeet per year

Inflow:

Underflow from outside the study areaMountain-front recharge -

Streamflow infiltration -

Tbtal 

Outflow:

Underflow leaving the study area

Discharge to streamflow 

Evapotranspiration 

Total 

13,000

800,000

1,700,000

2,500,000

130,000

300,000

2,100,000

2,500,000

1A11 values are rounded to two significant figures.

Streamflow infiltration was about 68 percent of the 

total inflow. More than 80 percent of the streamflow 

infiltration occurred along the Colorado River; most 

of the remainder occurred in the central and west basins along the Salt and Gila Rivers. Net recharge 

from streamflow infiltration was small in the southeast and highland basins because of shallow depths 

to water and the general presence of perennial flow 

in the major streams.

The quantity of mountain-front recharge that occurs 

in a basin is approximately proportional to the amount 

of precipitation that falls on the entire baein Mountainfront recharge was a principal inflow component in the 

basins in the southeast and highland groups and accounted for an estimated 25 and 55 percent, respectively, of the area-wide total. Recharge along the mountain

UNDERFLOW IN

STREAMFLOW

INFLOW

UNDERFLOW OUT 

FIGURE 8. Typical predevelopment inflow and outflow components in the alluvial basins.
A16 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

fronts was not significant in the Colorado River and 

west basins and was of minor significance in the central 

basins. Of the basins in these three groups, mountainfront recharge is largest in the Tucson basin, which is 

contiguous with the southeast group. Tucson basin exhibits some characteristics of the basins of the southeast group and some characteristics of the basins of the 

central group and could be included in either group 

depending on the particular factors that are being 

considered.

Recharge along the mountain fronts is a component 

of unknown magnitude in all basins but is important 

in steady-state and transient models of the hydrologic 

systems. A result of this study was the development of 

a relation between precipitation and mountain-front 

recharge. The average annual volume of mountainfront recharge was assumed to be related to average 

annual precipitation on the watershed. Average annual precipitation was used because data to document 

the quantity are readily available for entire basins. 

This component is used in spite of the fact that other 

precipitation characteristics, such as intensity, duration, magnitude, and temporal distribution, probably 

have a greater control on the quantity of recharge that 

occurs in any year. Other factors, including geology, 

land-surface slope, and vegetative cover, also were not 

considered but probably greatly influence the amount 

and spatial distribution of recharge.

The quantity of water potentially available for recharge is approximately equal to the precipitation 

minus evapotranspiration on the watershed. Part of 

the precipitation accounts for changes in soil moisture, which are assumed to be small over a long period. In addition, part of the precipitation leaves the 

basin as surface runoff, at least during periods of 

greater-than-average rainfall.

A regression equation was established between the 

average mountain-front recharge (Qrech) an(^ the total 

annual volume of precipitation on the watershed 

when the precipitation (P) is in excess of 8 in./yr 

(P>8). The equation was initially based on available 

previous estimates of recharge in a few basins (Halpenny and others, 1952; Anderson, 1972; Freethey, 

1982). By using this equation, the average annual 

mountain-front recharge to each basin in the study 

area was estimated. These estimates were then used 

in the water budgets of the individual basins. Adjustments were made to balance the water budgets of the 

individual basins and the water budget of the entire 

study area. After an iterative process of balancing 

and modifying, the resulting mountain-front recharge 

estimates were of similar magnitude and, with a few 

exceptions, appeared reasonable for basins with similar physiography and hydrology. The final equation

used to estimate the mountain-front recharge from 

precipitation is

log Qrech =-1.40+ 0.98 log P, (1)

where

P = precipitation greater than 8 in./yr.

The equation represents a means of approximating 

the total annual volume of mountain-front recharge 

to an entire basin. The regression equation is calibrated on the basis of recharge values from numerical models developed during this study (Anderson 

and Freethey, 1993), previous estimates of recharge 

in a few basins, and water budgets for the individual 

basins and the entire study area. No adjustments 

were made for variations in geology, vegetation, and 

other factors. Flow-model results indicate that simulated water levels are only slightly different using a 

uniform distribution of mountain-front recharge compared with intermittently spaced recharge (Anderson 

and Freethey, 1993).

Use of the average annual volume of precipitation 

that is greater than 8 in./yr on the entire watershed 

yielded better estimates of mountain-front recharge 

than using the total volume of precipitation (fig. 9A, 

B). The correlation index (R) for the regression is 

0.95 using precipitation rates greater than 8 in./yr 

and is 0.85 using the total precipitation. The threshold value below which little or no recharge may occur 

was set arbitrarily at the precipitation level of 8 

in./yr. Use of this threshold value effectively subtracts an amount of precipitation that is lost to soilmoisture deficits and evapotranspiration, especially 

the localized storms of short duration.

If the water budget of a basin is small, values of 

individual flow components also are small. Estimates 

of various flow components can be influenced greatly by 

the degree of uncertainties of the flow component. For 

example, the 95-percent confidence interval of the regression equation diverges rapidly when the mountainfront recharge is less than 1,000 acre-ft/yr (fig. 9B). 

This divergence is due to a lack of data and a high degree 

of uncertainty associated with a small water-budget 

component and small precipitation values.

Mountain-front recharge is independent of groundwater development in a basin. The only factor that 

could affect the quantity of mountain-front recharge 

is surficial alterations in the mountain areas that 

would result in a change in runoff. Climatic cycles 

affect the amount and timing of this recharge. The 

regression equation does not include factors such as 

geology, land slope, vegetation, and soil type that
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A17

may need to be considered for any future refinement 

of mountain-front recharge estimates.

Ground-water outflow from the aquifers occurs 

through three basic mechanisms: (1) discharge to 

springs and streams, (2) evaporation and transpiration from the water-table zone, and (3) underflow to 

adjacent basins (fig. 8). Outflow occurs at the downstream end of all basins; in a few basins, outflow also 

occurs along a large part of the length of the major 

stream that drains the basin as a combination of 

streamflow and evapotranspiration. The dominant 

mechanism through which outflow occurs depends on 

the individual geohydrologic setting within a basin 

and is greatly dependent on the physical setting at 

the downstream end of the basin.

The dominant outflow mechanism prior to development was evapotranspiration along stream channels 

(table 1), which consisted of evaporation from openwater and bare-soil surfaces and transpiration by 

riparian vegetation. The quantity of evapotranspiration was estimated on the basis of areal extent of 

vegetation determined from aerial photographs taken 

in 1935 and empirical values for average annual 

evaporation and consumptive use for the various

types of vegetation. Discharge along the Colorado 

River accounted for about 62 percent of the area-wide 

evapotranspiration component.

Downvalley flow from one basin to the next can 

occur as surface flow, as ground-water underflow, or 

as a combination of surface-water and ground-water 

flow. A surface-water component typically exists 

where the cross-sectional area of the aquifer is too 

small to transmit the total downvalley flow under the 

prevailing hydraulic gradient. For this study, groundwater underflow quantities were estimated with 

Darcy's law using the sparse information available on 

cross-sectional areas of flow, hydraulic gradient, and 

hydraulic conductivity of the alluvial material.

Discharge to streamflow in the highland basins accounted for about 91 percent of the prede velopment areawide component (table 1), with minor amounts occurring 

in the southeast basins (8 percent of the total) and west 

basins (1 percent of the total). Average discharge to 

streamflow was estimated on the basis of available 

records, including those of continuous-record gaging 

stations or periodic measurements of base discharge.

Ground-water underflow leaving the study area occurred in seven areas on the northwest and south

< tr i- <

QJ LU

SK

* *' LU 

LU LL

O

10'

o

en

10

10a

10 102 10 10 105

MOUNTAIN-FRONT RECHARGE, IN ACRE-FEET PER YEAR 

A. TOTAL PRECIPITATION ON THE BASIN

tr LU

LU *£

rr LU 

§> 

^tr

O LU

10'

TO

b LU 

o. tr

59 io5 LU <

tr z

LU (0 

O LU

z 10 4

CO

Z

<

10a

10

xa&o,*^

10* 10° 10 4

MOUNTAIN-FRONT RECHARGE (Qrech). 

IN ACRE-FEET PER YEAR

B. TOTAL PRECIPITATION GREATER 

THAN 8 INCHES ON THE BASIN

EXPLANATION

MOUNTAIN-FRONT RECHARGE VALUE USED 

IN NUMERICAL MODEL OF BASIN Shown 

only in B

MOUNTAIN-FRONT RECHARGE VALUE USED 

IN REGIONAL WATER BUDGET

FIGURE 9. Relation of estimated mountain-front recharge in a basin to total precipitation on the basin and total precipitation

greater than 8 inches.
A18 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

boundaries (Anderson and others, 1992, fig. 12). The 

estimated outflow quantity at six of these areas 

ranged from 500 to 3,800 acre-ft/yr, and the total represented less than 10 percent of the total underflow 

out of the area. At Yuma, underflow into Mexico was 

estimated to be more than 110,000 acre-ft/yr where 

the alluvial basin is transected by the international 

boundary.

An estimated 900 million acre-ft of recoverable 

ground water was stored in the upper 1,200 ft of alluvial deposits prior to development (Freethey and 

Anderson, 1986). The quantity of water stored in the 

alluvial-basin aquifers was hundreds of times greater 

than the quantity of water that flowed into and out 

of the basin aquifers annually. The ratio of the average annual inflow to a basin to the estimated volume 

of recoverable water stored in the upper 1,200 ft of 

that basin ranges from about 1:5 to 1:14,000. The 1:5 

ratio represents a basin with a small extent of basinfill deposits and therefore a small volume of water in 

storage, with a proportionally large annual inflow. 

The 1:14,000 ratio represents a basin in which annual inflow is small and the volume of water in storage 

is large. The typical range of the ratio for the major 

developed basins is from about 1:200 to 1:2,000.

Information used to estimate the volume of ground 

water in storage included the predevelopment watertable map (Freethey and Anderson, 1986), the depthto-bedrock map (Oppenheimer and Sumner, 1980, 

1981), and available information on the textural and 

water-bearing character of the various units in the 

aquifer systems (Evans and Haimson, 1982; Freethey 

and others, 1986; unpublished data from files of the 

U.S. Geological Survey). Specific-yield values assigned to the various units of the basin fill and 

stream alluvium ranged from 4 to 18 percent. The 

various parts of the aquifer system were summed, 

and the volume-weighted average specific yield 

ranged from 5 to 16 percent for entire basins. The 

arithmetic average for all basins was 11 percent.

Brown (1976) estimated the volume of recoverable 

ground water in storage for an equivalent area to be 

1,300 million acre-ft. The 900 million acre-ft derived 

in the Swab/RASA study is assumed to be more realistic because of the manner in which the geologic environment was discretized with variable specific-yield 

values assigned to individual units or parts of units 

on the basis of textural and water-bearing character. 

All other values, including area and saturated thickness, are assumed to be the same.

The quantities of recharge, discharge, and storage 

were estimated on the basis of field data or, in part, 

from previous studies. In general, for estimation of 

predevelopment quantities, the earliest available

data were used. However, measured ground-water 

levels and calculated hydraulic gradients may have 

been affected by development that had taken place 

when those data were collected. The estimates are 

representations of the likely order of magnitude of 

these quantities, and inaccuracies in the estimates 

must be considered. Although the confidence limits of 

these numbers are not known, they are assumed to 

be about ±25 percent.

DEVELOPMENT FLOW SYSTEM

Development of water resources in the area began 

in the late 1800's, first by diversion of surface water 

followed by pumping of ground water. Gradual changes occurred in the regional flow system from first development through 1980 and are continuing today.

Changes in inflow components have occurred 

through the alteration of streamflow infiltration and 

by the addition of recharge from reservoir and canal 

seepage and from irrigated fields. No reliable estimates of the quantities or change in quantities for 

these components have been made on a regional 

basis. Additional volume within which to store infiltrated streamflow has been provided in some basins 

because of water-level declines, and thus streamflow 

infiltration probably has increased locally. In other 

areas, however, streamflow infiltration probably has 

decreased because of upstream diversions and storage of surface waters.

The other inflow components of the predevelopment water budget are assumed to be unchanged. 

Underflow into the study area was a minor quantity, 

and any changes will not significantly affect the overall budget. Inflow occurs in an area remote from development and therefore is assumed to have been 

constant. Mountain-front recharge is proportional to 

rainfall and, although probably highly variable from 

year to year, is assumed to have not changed as a 

result of development.

Additional inflow to the ground-water systems has 

occurred in conjunction with the spatial redistribution 

of surface water and ground water, mainly through the 

use of these waters for irrigated agriculture. Recharge 

originates from seepage from surface reservoirs and 

canals and from infiltration of excess applied irrigation 

water. The quantity of recharge that originates through 

these sources is not well known and cannot be directly 

measured. Quantities have been estimated through indirect means such as water-budget analyses and 

ground-water flow models.

Increasing evidence is available to document the occurrence of recharge from percolation of part of the
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A19

applied irrigation water in excess of plant requirements. Areas where ground water is perched above the 

regional water table have been documented in at least 

three basins (Graf, 1980; Small, 1983; Cuff and Anderson, 1987) and anomalously high ground-water levels 

have been identified in another (Konieczki and English, 1979). Within the perched water zone, the aquifer material is saturated and the perched zone is 

separated from the regional ground-water system by 

an intervening unsaturated zone. The perched water 

zone may occur above the historical water level, indicating a change associated with development. Water in 

the perched zones typically is of poorer quality than 

the applied water because it contains the concentrated 

dissolved solids of the total volume of applied irrigation water, additional salts leached from the soil, and 

fertilizer and pesticide residue from surface applications (Schmidt, 1980). Model results also have provided evidence of recharge from excess applied irrigation 

water, although other factors also may be involved. 

Ten of twelve specific-basin flow models developed as 

part of this study required an additional amount of 

recharge in the agricultural areas to improve the 

match between observed and simulated heads (Anderson, 1983). The most consistent cause-and-effect relation, although not the only possibility, was that part of 

the applied irrigation water recharged to the regional 

ground-water system.

Significant changes have occurred in the outflow 

from most developed basins. Two major components 

of natural outflow evapotranspiration and discharge 

to streamflow were altered almost immediately in 

response to pumping. The major change in discharge, 

however, was the start of ground-water pumping in 

most basins. During 1915-80, pumpage in the study 

area totaled 184 million acre-ft (U.S. Geological Survey, 1982). The annual distribution of pumpage during that period is shown in figure 10. The disposition 

of the pumped water was predominantly consumptive 

use through evapotranspiration by agricultural vegetation; a part of the applied irrigation also percolated 

downward past the plant-root zone to become areal 

recharge. On the basis of the volume of sediments 

dewatered, approximately 50 percent of the total historical pumpage in the area represents a net depletion of ground-water storage (fig. 11). The other 50 

percent was replaced through reduction of other discharges, such as streamflow and evapotranspiration 

in riparian zones, or increases in recharge, such as

NET GROUND-WATER DEPLETION: 

ABOUT 50 PERCENT OF PUMPAGE

184,

ACREPUMPAGE, 

1915-80: 

000,000 

-FEET

FIGURE 11. Volume of water stored in the basin-fill 

aquifers to a depth of 1,200 feet below land surface, 

total volume pumped in 1915-80, and estimated net 

ground-water depletion.

FIGURE 10. Annual ground-water pumpage, 1915-80.
A20 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

increased streamflow infiltration and return of excess 

irrigation water.

The regional flow system was altered as a result of 

development. In 1980, discharge exceeded recharge 

in most developed basins. Ground-water flow rates 

and directions have been altered and the flow direction has been reversed in parts of a few basins. The 

result has been capture of part or all of former discharge components. The change in the basin flow system depends on the geohydrologic setting, which is 

one factor incorporated into the system of basin categorization for this study. The changes associated with 

water-resources development, therefore, are predictable and are related to basin category.

GEOCHEMISTRY

Geochemical information was analyzed to evaluate 

the ground-water quality for domestic and agricultural uses and to provide an independent insight into 

the hydrologic flow systems. The general chemical 

characteristics of ground water were documented, 

and geochemical models were developed to evaluate 

the reactions and mass transfer responsible for the 

evolution of ground-water chemistry. These models 

also provided a means of estimating the chemical 

character of ground water in undeveloped areas. The 

results of the geochemistry research aspect of this 

study are presented by Robertson (1991).

DISSOLVED SOLIDS AND OTHER CHARACTERISTICS

The dissolved-solids and trace-element contents of 

ground water in most basins are at suitable levels for 

most purposes. Water in a few basins, however, contains excessive dissolved-solids and trace-element 

concentrations and presents water-quality problems. 

Dissolved-solids concentrations range from about 300 

to 1,000 mg/L (milligrams per liter) throughout all or 

most of many basins but are from 1,000 mg/L to 

more than 10,000 mg/L in small parts of a few basins 

(Kister, 1973; Daniel, 1981; Thompson and others, 

1984). Chemical quality of ground water in the alluvial basins varies spatially and with depth and is 

closely related to geology, mineralogy, structure, internal and external drainage patterns, and historical 

development.

The areas of lowest dissolved-solids concentrations 

in ground water generally are near the mountain 

fronts, where recharge from surface runoff occurs. 

Concentrations typically range from 300 to 500 mg/L. 

Dissolved solids generally tend to increase in the

downgradient direction as a result of dissolution of 

minerals in the sedimentary deposits. Other chemical 

reactions, such as ion exchange and precipitation, 

also may occur and thus the dissolved-solids content 

may vary widely. Dissolved-solids concentrations generally exceed 1,000 mg/L along part of the lower 

reach of the Colorado River and along the Gila River 

from near Safford to its mouth near Yuma; in a few 

places along the lower Gila River, dissolved-solids 

concentrations exceed 10,000 mg/L (Daniel, 1981). 

Dissolved solids in ground water are elevated in 

these areas because of dissolution of soluble minerals 

within the alluvium or, in a few places, concentration 

by evapotranspiration (Robertson, 1986a).

Generally, dissolved solids in ground water greater 

than 1,000 mg/L indicate that gypsum, anhydrite, 

halite, and other soluble salts are present in the 

basin-fill aquifers. In an 80-square-mile area that includes the Willcox Playa in southeastern Arizona, 

ground water contains from 1,000 to as much as 

43,000 mg/L of dissolved solids, a result of internal 

surface-water drainage and ground-water discharge 

unique to the playa. Other localized areas where 

ground water contains more than 1,000 mg/L of dissolved solids are in the Benson, Douglas, Tucson, 

Eloy, Stanfield, Safford, and Waterman Wash basins; 

in Verde, McMullen, and east and west Salt River 

Valleys; in Ranegras Plain; and along the Colorado 

and lower Gila Rivers (Thompson and others, 1984; 

Robertson, 1990). Most of these areas are of small 

areal extent and probably reflect the dissolution of 

disseminated evaporites from within the aquifer 

material. Depth and producing interval are not 

known for all the wells that were sampled, but wellconstruction characteristics may influence dissolvedsolids concentrations measured in water samples.

Dissolved-solids concentrations in some basins increase with depth in the upper 2,000 ft and in other 

basins decrease with depth. No statistically significant trend can be demonstrated for the entire study 

area. In most basins, however, concentrations of magnesium, bicarbonate, and silica decrease with depth, 

while pH, temperature, and concentrations of most 

trace elements increase with depth.

The temperature of ground water generally ranges 

between 15°C and 30°C in recharge areas and increases to as high as 45°C near discharge areas. The 

pH values of ground water in recharge areas generally are near neutral but increase in downgradient and 

discharge areas to values as high as 9.5. Dissolved 

oxygen is near saturation in ground water in recharge areas and typically decreases to 50 to 80 percent of saturation near discharge areas (Robertson, 

1991).
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A21

GROUND-WATER CHEMISTRY TYPES

The chemistry of water in the basin-fill aquifers can 

be characterized by type on the basis of the relative 

concentrations of dissolved cations and anions. General patterns exist in the distribution of water types. The 

mineral composition of the rocks of the mountains and 

the unsaturated zone and the rock-water reactions influence the chemical type of ground water.

Calcium bicarbonate or calcium sodium bicarbonate type ground water is prevalent near the 

mountain-front recharge areas of most basin groups. 

The ground water evolves to a sodium mixed-anion 

type near the center or near the outlet of some basins 

or, as dissolved solids increase, to a sodium chloride 

or sodium calcium chloride sulfate type. Water that 

contains dissolved-solids concentrations of 500 mg/L 

or less is a sodium mixed-anion type. Ground water is 

a sodium chloride type in the Willcox Playa and in the 

east and west parts of the Salt River Valley. In areas 

where dissolved-solids concentrations exceed about 

1,000 mg/L, such as along the Gila River and the 

Colorado River, ground water is predominantly a sodium chloride or sodium calcium sulfate type. Ground 

water in the alluvial aquifers of the basins in the 

highland group is mainly a calcium bicarbonate or 

calcium magnesium bicarbonate type, particularly 

along the many streams. A calcium sodium bicarbonate or sodium calcium bicarbonate type water also 

occurs in parts of the southeast basins where recharge occurs along the valley floors.

TRACE ELEMENTS

In several basins, ground water contains concentrations of trace elements that exceed standards for 

public supply and certain agricultural uses. Fluoride, 

barium, chromium, arsenic, nitrate, mercury, lead, 

and boron are known to present environmental problems in some areas where concentrations exceed the 

maximum contaminant level (MCL) established by 

the U.S. Environmental Protection Agency (1977) for 

drinking water. The sources of these trace elements 

are the basin-fill sediments, which were derived from 

igneous, metamorphic, and sedimentary rocks of the 

surrounding mountains. In general, trace elements 

occur in concentrations below the detection level in 

the zones of mountain-front recharge, where a large 

percentage of the ground water enters the aquifer 

system. Concentrations tend to increase in the downgradient direction.

In many basins, ground water contains fluoride concentrations that exceed the MCL. The ingestion of

water containing small concentrations of fluoride is 

beneficial in the prevention of tooth decay, but large 

concentrations can result in mottling of teeth. Concentrations of fluoride greater than 6 mg/L are closely 

related to the occurrence of rhyolitic and andesitic volcanic rocks, which appear to serve as a major source 

of fluoride. Where granite and gneiss are the dominant rock types, fluoride concentrations generally are 

less than 1 mg/L but in places may exceed 3 mg/L.

Fluoride concentrations exceed 2 mg/L in small 

parts of many basins and exceed 5 mg/L in parts of 

the confined basin-fill aquifers of the Benson, lower 

San Pedro, and Safford basins and San Simon Valley. 

Concentrations also exceed 5 mg/L in large parts of 

Ranegras Plain; lower Hassayampa, Waterman 

Wash, and Gila Bend basins; and the Sentinel Plain 

area west of the Gila Bend basin. The largest fluoride concentration, which was in excess of 200 mg/L, 

is in the Willcox Playa and is a result of evaporative 

concentration (Robertson and Garrett, 1986).

GEOCHEMICAL MODELS

Geochemical models can be used to simulate chemical reactions that occur between water and rock material of the aquifer as the water moves along a flow 

path through the hydrologic system. The purpose of 

geochemical modeling is to quantify the natural 

geochemical processes that control the chemistry of 

ground water. Data used in the modeling include the 

chemical composition of water in the recharge area, 

mineralogy of the rock matrix that makes up the 

aquifer, and chemical composition of ground water at 

one or more points along a flow path or at a point of 

discharge from the basin. These models provide information to evaluate reactions occurring in the system 

and an independent means of analyzing the hydrologic flow system. Geochemical models were developed for six basins (fig. 6). The model results were 

used to verify an existing conceptual model of the 

flow system and provided valuable insight into the 

relative significance and magnitude of various flow 

components. Robertson (1991) grouped the basins of 

the study area into two types open systems and 

closed systems on the basis of the chemical evolution as derived from geochemical-model results.

Open systems are those in which additional water 

and gases may be added to ground water along the flow 

path within a basin. These conditions prevail where 

precipitation is greatest, where shallow ground water 

occurs, and where no extensive fine-grained unit overlies the principal basin-fill aquifer. These conditions 

occur in the highland basin group, in the water-table
A22 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

aquifers of the southeast basin group, and in parts of 

the central basin group. Specific basins of this type 

include Willcox, Douglas, upper San Pedro, and Tucson 

basins and Avra and Altar Valleys. Open systems occur 

in the shallow aquifers that include stream alluvium 

along the Gila, Salt, and San Pedro Rivers and San 

Simon Creek and in the Big and Little Chino Valleys, 

Agua Fria basin, and other stream-alluvium aquifers 

of the highland basin group. Because of the contributory 

effects of additional water and gases along the general 

flow path, geochemical modeling and predictability of 

open systems are not as reliable as those of closed 

systems unless large dissolved-solids concentrations 

are present. Large dissolved-solids concentrations 

occur mainly from dissolution of halite and gypsum, 

and these reactions can be predicted.

Closed systems are those that do not receive additional recharge or gases along the flow path. Closedsystem conditions exist where depths to ground 

water are great and recharge occurs only along the 

mountain fronts. These conditions also exist in confined-aquifer systems or where leaky confined conditions occur in which hydraulic head increases with 

increasing depth. This type of system occurs in the 

west basin group, in most of the central basins, and 

in the deep confined aquifers of lower San Pedro, 

Benson, San Bernardino, and Safford basins and Little Chino and San Simon Valleys. In closed systems, 

the chemistry of ground water is determined by the 

reactions of the initial recharge water with the minerals present in the aquifer matrix as the water 

moves downgradient. In such a system, the concentrations of major dissolved ions and the occurrence of 

some trace elements can be reliably predicted.

The results of geochemical modeling indicate that 

the processes that control ground-water chemistry include weathering of plagioclase, potassium feldspar, 

and ferromagnesian minerals; formation of montmorillonite; precipitation of calcite and probably of silica; and, in some basins, ion exchange (Robertson, 

1989a). On the basis of these reactions and the associated mass transfer, information transfer was tested 

and successfully applied in some basins and is a potentially powerful tool for assessment of water quality and geochemistry in undeveloped basins.

WATER-RESOURCES DEVELOPMENT

HISTORY OF DEVELOPMENT

Archeological evidence indicates that development 

of water resources in part of the area began as early 

as 300 B.C. when the Hohokam Indian culture developed an agricultural-based society and diverted waters of the Salt and Gila Rivers through a network of 

canals to irrigate the flat land adjacent to the rivers 

(Haury, 1976; Lister and Lister, 1983). In the middle 

to late 1800's, as part of the westward migration, 

early settlers used available surface waters to support agricultural development. Because of the sporadic nature of streamflow in the area, ground-water 

resources were soon developed to supplement the 

highly variable surface-water supply. By 1889, the 

total irrigated acreage in Arizona was nearly 66,000 

acres (Davis, 1897, p. 54).

The first wells in the area generally were shallow, 

hand-dug wells of large diameter. In the late 1800's 

and early 1900's, most wells were along the flood 

plain of perennial streams. Because centrifugal 

pumps were the type commonly available at that 

time for lifting large quantities of water, pumping for 

irrigation generally was in areas of shallow ground 

water. In 1915, an estimated 123,000 acre-ft of 

ground water was pumped from wells (U.S. Geological Survey, 1982), mainly near Florence and Coolidge 

in the Eloy basin and near Phoenix in the Salt River 

Valley.

Development began at different times in various 

basins. As early as 1920, development was mostly in 

the east and west Salt River Valley, Eloy basin, and 

Stanfield basin (fig. 1). In 1940, nearly 1 million 

acre-ft of ground water was withdrawn in the east 

and west Salt River Valley; another 0.5 million acreft was withdrawn in Eloy, Stanfield, Tucson, and 

Yuma basins. The remainder of the basins were undeveloped or slightly developed until after 1940.

Several factors greatly affected the use and development of the ground-water resources. Development 

of the deep-well turbine pump in the 1930's increased 

the accessibility of ground water. Availability of 

cheap electric power aided in the growth of groundwater use. Increased demands for agricultural products linked with technological advances resulted in 

vast increases in pumpage during and after the 

1930's. During World War II, the most substantial increase was in the acreage of cotton grown in support 

of the war effort.

Since World War II, the basin-fill aquifer systems 

have been increasingly stressed to support agricultural activities. The estimated pumpage in 1942 was 

about 1.7 million acre-ft (U.S. Geological Survey, 

1982; fig. 10, this report) and was the beginning of 

a period of rapid growth. By 1952, the estimated 

pumpage had more than doubled to 3.8 million acreft, and by 1962 pumpage was about 4.8 million acreft. During 1950-80, average pumpage was estimated 

to be 4.8 million acre-ft/yr, which is nearly twice the
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A23

average annual recharge into the area. The volume 

of pumpage in a basin in excess of recharge represents withdrawal from storage. Pumpage is estimated to range from about 2 to more than 200 times 

greater than the recharge rate in individual basins. 

Total irrigated acreage in the study area in 1980 

was about 1.2 million acres; the major crop was cotton, which accounted for about 50 percent of total irrigated acreage. Other extensively grown crops were 

grains and hay, which together accounted for 42 percent of the irrigated acreage In 1980, crop production from irrigated agriculture accounted for about 7 

percent of the area's economic income (Valley National Bank, 1983). Agriculture accounted for 88 percent 

of the area's total water use in 1980 (White and 

Anderson, 1985; Anderson and White, 1986).

EFFECTS OF DEVELOPMENT

The effects of development of water resources on 

the hydrologic systems can be related to basin category. The dominant effect, which has occurred in 

nearly all basins, is decline of water levels and the 

associated depletion of aquifer storage, although 

water levels have risen in some basins along the Colorado River as a result of irrigation. This effect is 

directly measurable through monitoring of water levels. Other attendant effects include alteration of the 

natural flow system, which potentially can result in 

significant alterations of the natural environment; 

changes in water quality; and the occurrence of land 

subsidence and formation of earth fissures.

Large quantities of ground water are stored in the 

interstices of alluvial sediments that fill the basins. 

This water accumulated over thousands of years. Development has resulted in removal of part of that water 

and, where pumping exceeds the natural rate of recharge, overdraft or mining of ground water has occurred. Declining water levels and depletion of aquifer 

storage are one effect of development and, for a unit 

volume of water pumped from an aquifer, part or all 

of that volume can represent a net removal from storage. The aquifer response depends on the hydrologic 

setting primarily the potential for capturing additional recharge or diverting natural discharge. The 

magnitude of water-level declines also depends on the 

geologic setting specifically, lithology and texture of 

the basin-fill deposits. For a unit volume of pumpage, 

water levels decline more near the center of pumpage 

in a setting of fine to very fine grained sediment than 

in a setting of coarse-grained sediment.

From the time development began through 1980, the 

maximum water-level decline in the area was more

than 450 ft in Stanfield basin; during 1923-77, more 

than 420 ft of decline occurred southeast of Chandler 

in the east Salt River Valley (Laney and others, 1978). 

Declines of more than 200 ft occurred by 1980 in Harquahala Plain, McMullen Valley, Willcox basin, San 

Simon Valley, Eloy basin, and throughout large areas 

of the east and west Salt River Valley (fig. 1). The 

approximate extent and magnitude of water-level declines in the study area from the time development 

began through about 1980 are documented in figure 12 

(Anderson and others, 1992, pi. 3).

The greatest alteration of a ground-water flow system occurred in Stanfield basin in the central part of 

the study area. Ground-water flow through the area 

in 1923 was from the southeast toward the northwest 

(fig. 13A). The gradient of the water table ranged 

from about 5 to 20 ft/mi (Anderson, 1968). By 1977, 

more than 450 ft of water-level decline had occurred 

and ground-water flow was approximately radial toward a cone of depression centered on the west side 

of the basin (fig. 135). The gradient of the water 

table was as great as 100 ft/mi toward the center of 

the cone of depression (Konieczki and English, 1979), 

and the direction of flow in the northern part of the 

basin changed from northwest in 1923 to south or 

southwest by 1976-77.

Rates of water-level decline have ranged from less 

than 1 ft/yr in areas of slight development to nearly 

24 ft/yr as measured at one well in Stanfield basin in 

the 1950's (Anderson and others, 1992, fig. 26A). Decline rates generally have been less since the late 

1960's than they were during the 1950's to mid1960's. The most important factor is decreased pumpage in areas of maximum decline. This decrease in 

pumpage probably is related to increased cost of 

power compounded by increased power requirements 

per unit of water pumped because of water-level declines. Increased capital investment needed for deepening wells and installing equipment of greater lift 

capacity also could be a related influence. Other factors are increased recharge from infiltration of excess 

applied irrigation water and drainage of water from 

fine-grained units as a result of inelastic compaction. 

Inelastic compaction results in a permanent loss of 

ground-water storage. Increased irrigation efficiency 

and conversion to crops that require less water could 

be locally significant but are not the cause of the general, widespread trend of decreased rate of waterlevel decline.

The volume of ground-water depletion in developed 

basins has ranged from less than 1 percent to as 

much as 15 percent of the estimated volume that was 

in storage within 1,200 ft below land surface prior to 

development. A small increase in ground-water
A24 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

109° 108°

31

100 KILOMETERS

EXPLANATION

WATER-LEVEL

DECLINE, IN FEET

0-50

50-200

Greater than 200

ALLUVIAL DEPOSITS Locally may include 

evaporite deposits and volcanic rocks

BEDROCK OF THE MOUNTAINS 

BASIN BOUNDARY 

BOUNDARY OF STUDY AREA

FIGURE 12 Areas of water-level decline from the beginning of development through 1980.
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A25

112° 111M51 112

32°45' -

R. 2 E. R. 3 E. R. 4 E. R. 5 E. R 6 E

From Anderson (1968)

A.1923

32°45

R. 2 E. R. 3 E. R. 4 E. R. 5 E. R . 6 E.

From Konieczki and 

English (1979)

B.1976-77

10

I

20 MILES

I

\ 

20 20 KILOMETERS

1:250,000 

EXPLANATION

BEDROCK OF THE MOUNTAINS

APPROXIMATE EXTENT OF MAIN 

WATER-BEARING UNIT

 900-

DIRECTION OF GROUND-WATER FLOW 

BOUNDARY OF STANFIELD BASIN

WATER-TABLE CONTOUR Shows altitude 

of water table. Contour interval 50 and 

100 feet. Datum is sea level

109°

INDEX MAP SHOWING LOCATION OF

STANFIELD BASIN (SHADED) IN

Swab/RASA STUDY AREA

0 50 MILES

 

0 50 KILOMETERS

FIGURE 13. Ground-water flow system in Stanfield basin, 1923 and 1976-77.
A26 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

storage has occurred in several basins along the Colorado River following several feet of water-level rise 

under irrigated fields.

As ground water is withdrawn from storage and 

water levels decline, additional system adjustments 

also occur. Withdrawal and consumptive use in a 

basin have resulted in capture of all or part of former 

discharges and has increased recharge. Specifically, 

evapotranspiration and underflow have decreased in 

a number of basins. Recharge from infiltration along 

major streams generally has increased, but the areas 

where increases have occurred are only along those 

streams where a significant part of the base flow has 

been diverted, either by upstream withdrawal or by 

ground-water pumping. In some instances, underflow 

into a basin has been increased, although this depends greatly on development in the upstream basin. 

Once development begins, the trend is for outflow to 

decrease because of partial capture by ground-water 

pumping. The overall result can be a change in environment, particularly in the riparian zone. A streamflow regimen change from perennial to intermittent 

will result from upstream diversion and groundwater pumping and is the most readily observed effect. This change has occurred throughout the reach 

of the Salt River from Granite Reef Diversion Dam to 

near the confluence of the Salt and Gila Rivers west 

of Phoenix. Extensive reaches of the Gila River between Safford and Yuma have undergone major 

changes in flow regime because of upstream diversion 

and storage. Near Tucson, selected reaches of the 

Santa Cruz River and Tanque Verde and Rillito 

Creeks no longer contain perennial flow because of 

extensive ground-water development and associated 

lowering of the water table.

Riparian habitat has been severely altered as 

ground-water levels are drawn down below the root 

zone of the riparian vegetation. Extensive mesquite 

bosques have died because of this change in the hydrologic system. Instances where this has occurred 

are documented along the Gila River near Coolidge 

in Eloy basin, west and southwest of Tucson along 

the Santa Cruz River, and along the Salt River east 

of Phoenix. Decreases in acreage of riparian vegetation and annual consumptive use from 1952 to 1967 

were estimated to be 120,400 acres and 435,000 acreft, respectively, as a result of ground-water development (Arizona Water Commission, 1975, p. 107).

In some areas, effects of ground-water mining have 

been lessened by the capture of additional recharge 

or natural discharge. Opportunities for this capture 

to occur generally were of minor significance or insignificant in the west basins, and were moderately significant in the central basins, especially after flows of

the major rivers the Gila, Salt, Verde, and Agua 

Fria were diverted or stored in upstream reservoirs. 

Opportunities for significant capture existed in the 

southeast and highland basins. Almost total capture 

has occurred in Willcox and Douglas basins. In all 

other basins of the southeast and highland groups, 

natural discharge by evapotranspiration and streamflow continues, although reductions in these flow 

components have occurred. The hydrology of the Colorado River basins is dominated by surface flow in 

the river, which acts as a source or sink for water in 

the basin-fill aquifers. Capture of discharge is affected by the rapid establishment of a new equilibrium 

condition following an increase or decrease in 

ground-water pumping nearly all the change in the 

flow system involves the quantity of ground-water 

flow to or infiltration from the river.

Water-level declines reflect the change from saturated to unsaturated conditions in the aquifer material 

and are accompanied by increases in the intergranular 

loading in the sediment because of the removal of the 

buoyant effect of the water. The result is slight elastic 

compaction of the coarse-grained aquifer material and 

greater inelastic compaction in the fine-grained sediment owing to reduction of the volume of pore space. 

The degree of inelastic compaction is related to thickness and inelastic compressibility of the fine-grained 

sediment contained in the aquifer system. In basins 

where the aquifer system includes thick sequences of 

compressible fine-grained sediment, land subsidence 

can occur in response to dewatering of the sediment or 

decline of artesian head in the deeper, confined parts 

of the system. Because the basins are surrounded and 

underlain at variable depths by consolidated rocks and 

the subsurface lithology is not uniform, vertical subsidence does not occur uniformly over large areas. Tensional stresses are created near the perimeter of areas 

of large water-level decline and land subsidence (Laney 

and others, 1978). The result is the occurrence of earth 

fissures in these areas of greatest tensional stress.

Land subsidence and the occurrence of earth fissures, which are related to ground-water withdrawal 

and associated water-level declines, have been documented in highly developed parts of central Arizona 

(Schumann and Poland, 1970). Near Eloy, subsidence 

measured at one bench mark during 1952-77 was 

12.5 ft (Laney and others, 1978). In Eloy and Stanfield basins, more than 120 mi2 are known to have 

subsided at least 7 ft during 1952-77 (Laney and 

others, 1978), and more than 1,000 mi2 are known to 

have subsided at least 3 ft (Poland, 1981). During 

1937-74, more than 5 ft of subsidence was documented at one bench mark about 8 mi northwest of 

Willcox, and 4.6 ft of subsidence was measured dur-
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A27

ing 1952-74 near Bowie in San Simon Valley (Holzer, 

1980). Earth fissures have been mapped at various 

times since they were noted in 1927 (Leonard, 1929), 

and a recent study (Schumann and Genualdi, 1986) 

documented earth fissures in southern Arizona. Fissures start as narrow linear features and increase in 

width and length as additional ground movement 

and erosion occur. Vertical offset may occur across 

the earth fissure. A fissure system near Eloy (fig. 14) 

is more than 8 mi long and has had vertical offset of 

about 3 ft (Winikka and Wold, 1977).

The principal effect of subsidence on basin hydrology is the permanent loss of aquifer storage. The volume of lost storage in the aquifer is equal to the 

volume of land subsidence, which in the Eloy basin 

alone is nearly 1 mi3 or about 3.4 million acre-ft of 

water. Generally, inelastic compaction of fine-grained 

sediment continues for a long time following a reduction in pumping. On the basis of one-dimensional 

compaction modeling, Epstein (1987) estimated that 

the sediment near Eloy would continue to compact 

for more than 300 years even if withdrawals ceased. 

Expulsion of additional water that does not discharge 

through gravity drainage accompanies compaction of

the fine-grained sediment. This water of compaction 

contributes to the decreased rate of water-level decline observed since the late 1960's.

The geohydrologic environment in the alluvial basins is conducive to the occurrence of land subsidence. 

Because of the common presence of extensive finegrained sediment in the center of most basins, groundwater depletion and the attendant increase in applied 

system stress will result in inelastic compaction of the 

sediment after a threshold value of water-level decline 

is exceeded. This threshold value varies as a function 

of geologic environment but is typically on the order of 

150-200 ft. Land subsidence is a problem in several 

basins and is a potential problem in all basins where 

ground-water withdrawals will result in water-level 

declines greater than about 150 ft.

Development of techniques to simulate land subsidence has been ongoing as a followup to the current 

RASA study. A compaction simulation program has 

been developed, tested, and documented as part of 

that followup study (Leake and Prudic, 1988). Simulation and model calibration were conducted using data 

from the Eloy basin and have proved successful. The 

ability to simulate and predict subsidence will become

FIGURE 14. Earth-fissure system more than 8 miles long, near Eloy, Arizona.
A28 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

increasingly important as an adjunct to successful 

management of ground water. Additional work is needed to monitor actual land subsidence and earth fissuring in the most extensively developed basins and to 

collect detailed information on the basin lithology in 

order that subsidence can be included as an integral 

part of future simulation studies.

The physical effects of land subsidence and earth 

fissuring are of economic importance in developed 

basins. Changes in land-surface slope, especially 

along the outer area of subsidence, have resulted in 

damage to irrigation systems, sewer systems, pipelines, streets, roads, interstate highways, and railroads. A common problem caused by compaction is 

the collapse of well casings. More extensive flooding 

from surface runoff may result from changes in the 

channel slope because of subsidence. In a few places, 

effects of land subsidence and earth fissuring have 

necessitated rerouting the Central Arizona Project 

canal, which is used to import water from the Colorado River to the central part of Arizona. Monitoring 

and investigations of subsidence are essential to develop methods of predicting the occurrence and magnitude of subsidence and earth fissures.

A general trend exists in most basins in which the 

sediments are more consolidated and more strongly 

cemented with increasing depth. The result generally 

is decreased well yield with depth. Ground-water quality also is variable with depth in many basins and 

reflects the variable composition of basin-fill sediment. 

In areas where evaporites are present, dissolved solids 

in ground water may increase severalfold in the downgradient direction and water type likely will change 

from a calcium bicarbonate or sodium bicarbonate type 

to a sodium chloride or sodium calcium sulfate chloride 

type.

A major effect of development on water quality is 

associated with the migration of chemicals from land 

surface to the water table with accompanying adverse 

effect on the quality of ground-water supplies. Chemical 

contaminants have been detected in the unsaturated 

zone, in perched water zones, and in the saturated zone. 

The chemicals include a broad array of agricultural 

fertilizers and pesticides, industrial solvents and trace 

metals, and chemicals associated with municipal and 

industrial waste. Both nonpoint and point sources have 

contributed to ground-water contamination. The common means for chemicals to reach the water table has 

been by leaching through the unsaturated zone following surface application or shallow burial. Contamination has occurred by rapid movement of chemicals 

through well boreholes and through preferential flow 

paths such as animal burrows, plant-root holes, small 

vertical cracks, or more permeable zones that occur

naturally in the alluvial deposits. Earth fissures also 

represent a means of solutes reaching the water table 

with little or no attenuation.

The extent of degradation of the ground-water 

quality attributable to the percolation of chemicals 

from the land surface depends on the volume and 

rate of flow through the unsaturated zone. Few data 

are available to document the rate of flow. In undeveloped basins, moisture content in the thick unsaturated zones is small. Therefore, the thickness of 

unsaturated material and the difference between 

moisture content of the sediment prior to development and at field capacity will determine the amount 

of infiltration necessary and the time required before 

recharge will occur at the regional water table.

Stream alluvium along the major surface drainages 

in the area serves as efficient natural recharge areas. 

The physical nature of the alluvial material has 

aided certain adverse effects of development. The unconsolidated, surficial materials are easily mined, 

and the well-sorted sand and gravel fulfills a broad 

range of construction-related uses. Sand- and gravelmining operations have affected the rate and volume 

of natural sediment transport in streams during 

major runoff events. In addition, the large open pits 

that result from gravel mining are attractive locations to bury municipal and industrial wastes. Liquids readily move through the stream alluvium, and 

contamination of ground water has occurred because 

leachate has drained from these waste-disposal sites. 

Chemicals in the landfills also may be dissolved and 

flushed through the material by water that infiltrates 

during flood events. Present and future activities in 

these major recharge areas may have a significant effect on regional water quality.

CONSIDERATIONS FOR FUTURE DEVELOPMENT

The responses of the ground-water systems to development have been documented and follow a systematic 

pattern related to the geohydrologic characteristics of 

the five basin categories defined previously. The general 

relation between increased pumpage and changes in 

the principal water-budget components for each basin 

category is shown in figure 15.

The relation between magnitude of pumpage and 

net volume of water withdrawn from storage is illustrated in figure 15A. An extreme condition occurs in 

the west basins, where pumpage of almost any magnitude will result in storage depletion; the relation is 

virtually linear. The other extreme is represented by 

the Colorado River basins. Only a small volume of 

water can be withdrawn from aquifer storage as
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A29

A. VOLUME OF GROUND WATER 

FROM STORAGE

INCREASING VOLUME REMOVED FROM STORAGE

B. AQUIFER DISCHARGE THROUGH 

EVAPOTRANSPIRATION

INCREASING DISCHARGE THROUGH EVAPOTRANSPIRATION

C. AQUIFER DISCHARGE TO STREAMS

NOTE: No discharge to 

streamflow in west 

basins

INCREASING DISCHARGE TO STREAMS 3>

FIGURE 15. Generalized relation between increasing pumpage 

and changes in water-budget components.

pumpage increases from aquifers of this basin group 

as long as surface water in the Colorado River is 

available to infiltrate into the ground-water system 

and replace the withdrawn water. When water is 

pumped from a well, the gradient from the river toward the well is increased and additional infiltration 

of river water results. The typical response of the 

other basin groups range between these two extremes, depending partly on the rate at which capture of other discharge and recharge components 

occurs.

Evapotranspiration by native vegetation typically 

decreases as nearby pumping increases and water 

levels decline. The general system response for each 

basin group is illustrated in figure 15B. A small 

change occurs in basins of the Colorado River group, 

and conversely all natural discharge by evapotranspiration is captured soon after pumping is initiated in 

the basins of the west group. Similarly, changes in 

discharge to streamflow in response to pumping are 

illustrated in figure 15C. The Colorado River group is 

again representative of one extreme, indicating only 

a small change in aquifer discharge to streams. Because no stream base flow generally occurred in the 

west basins, except for what occurred in the lower 

Gila River prior to development, no change can occur 

in the typical west basin. In basins of the central 

group, the small quantity of water that discharged to 

streams was rapidly and totally captured following 

the start of pumping. These two system responses 

decreases in natural evapotranspiration and discharge to streamflow are functionally related. 

Ground-water development near the stream flood 

plain eventually will result in capture of part or all 

of these flow components, which will alter the natural riparian environment.

Development through 1980 depleted from less than 

1 to about 15 percent of ground water in storage in 

the alluvial-basin aquifers; a much larger volume remains in storage. The degree and rate at which further depletion occurs will be controlled by economics. 

As in the past several decades, future growth probably will occur at the expense of agriculture.

Although the magnitude of the available groundwater supply will not be reduced as contamination 

occurs, the expense of treating contaminated waters 

may serve that effect. Additional studies and monitoring are needed to determine the effects of areal 

application of chemicals and burial of wastes. Alternatives to present disposal techniques need to be 

found. Various conservation and public-education 

campaigns presently are being pursued.

The Arizona Groundwater Management Act of 

1980, which was enacted to reduce the long-term
A30 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

overdevelopment of ground-water resources, will significantly affect future development. As a result of 

this act, use of water for agriculture will be systematically reduced and financial constraints will be 

used to encourage efficiency in water use. Management alternatives will look at increasing the water 

supply through various augmentation schemes and 

decreasing the water demand through conservation 

and water-use pricing. The long-term goal in three 

areas of greatest overdraft is to reach a "safe yield" 

condition, in which withdrawal is balanced by recharge, by the year 2025 (Arizona Groundwater Management Study Commission, 1980).

The inequality of supply and demand resulted in 

ground-water mining from 1940 to 1980 and continues today (1993). In an attempt to bring the supply 

side of the equation into better balance with the demand side, water from the Colorado River has been 

imported to the central part of Arizona since 1985. 

Other methods of augmentation are being studied 

and include increasing natural recharge, artificial recharge, watershed management, and cloud seeding. 

At the same time, decreasing the demand side of the 

equation is being pursued. Approaches being studied 

include increased irrigation efficiency, including 

growing crops that consume less water, and the use 

of level-field and drip irrigation; conservation measures; and reuse of sewage effluent and other water 

of impaired quality by industry and the public.

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Survey Water-Supply Paper 2, 98 p.

DeCook, K.J., and Wilson, L.G., 1980, Ground-water recharge from 

urban runoff and irrigation returns, in Proceedings of the 

Deep Percolation Symposium, May 1-2, 1980: Arizona Department of Water Resources Report 1, p. 37-52.

Eaton, G.P., Peterson, D.L., and Schumann, H.H., 1972, Geophysical, geohydrological, and geochemical reconnaissance of the 

Luke Salt body, central Arizona: U.S. Geological Survey Professional Paper 753, 28 p.

Epstein, V.J., 1987, Hydrologic and geologic factors affecting land 

subsidence near Eloy, Arizona: U.S. Geological Survey WaterResources Investigations Report 87-4173, 28 p.

Evans, L.G., and Haimson, J.S., 1982, Swab/RASA aquifer parameter 

study: Arizona Department of Water Resources, final report for 

U.S. Geological Survey contract 14-08-0001-18268, 21 p.

Fenneman, N.M., 1931, Physiography of western United States: 

New York, McGraw-Hill, 534 p.

Fennessy, P.J., Carter, K.G., and Keith, S.J., 1980, Summary of literature information on recharge in arid-semiarid basins: Tucson, University of Arizona, Water Resources Research Center 

report, 259 p.

Fields, R.L., 1986, Data-processing activities of the Southwest Alluvial Basins, RASA study, in Anderson, T.W, and Johnson, 

A.I., eds., Regional aquifer systems of the United States, 

Southwest alluvial basins of Arizona: American Water Resources Association Monograph Series 7, p. 17-23.

Fields, R.L., and Vetter, E.F., 1984, A data-management system for 

use in a ground-water modeling and resources evaluation environment: U.S. Geological Survey Water-Resources Investigations Report 84-4014, 227 p.

Freethey, G.W, 1982, Hydrologic analysis of the upper San Pedro 

basin from the Mexico-United States international boundary 

to Fairbank, Arizona: U.S. Geological Survey Open-File Report 

82-752, 64 p.

 1984a, Ground-water modeling, alluvial basins of Arizona, in 

Replogle, J.A., and Renard, KG., eds., Water today and tomorrow: Specialty Conference, Irrigation and Drainage Division of 

the American Society of Civil Engineers, Flagstaff, Arizona, 

July 24-26, 1984, Proceedings, p. 675-682.

1984b, Considerations in modeling ground-water flow in the alluvial basins of Arizona, in Anderson, T.W, and Johnson, A.I., 

eds., Regional aquifer systems of the United States, Southwest 

alluvial basins of Arizona: American Water Resources Association Monograph Series 7, p. 57-67.

Freethey, G.W, and Anderson, T.W, 1986, Predevelopment hydrologic conditions in the alluvial basins of Arizona and adjacent 

parts of California and New Mexico: U.S. Geological Survey 

Hydrologic Investigations Atlas HA-664, 3 sheets.

Freethey, G.W, Pool, D.R., Anderson, T.W, and Tucci, Patrick, 

1986, Generalized distribution of aquifer lithology in the alluvial basins of Arizona and adjacent parts of California and 

New Mexico: U.S. Geological Survey Hydrologic Investigations 

Atlas HA-663, 4 sheets.

Graf, C.G., 1980, Maps showing ground-water conditions in the 

Harquahala Plain area, Maricopa and Yuma Counties, 

Arizona 1980: Arizona Department of Water Resources, Hydrologic Map Series Report 1, scale 1:125,000, 3 sheets.

Halpenny, L.C., and others, 1952, Ground water in the Gila River 

basin and adjacent areas, Arizona A summary: U.S. Geological Survey open-file report, 224 p.

Harshbarger, J.W, Lewis, D.D., Skibitzke, H.E., Heckler, W.L., and 

Kister, L.R., 1966, Arizona water: U.S. Geological Survey 

Water-Supply Paper 1648, 85 p.

Haury, E.W, 1976, The Hohokam Desert farmers and craftsmen: 

Tucson, University of Arizona Press, 412 p.

Healey, D.L., 1970, Calculated in-situ bulk densities from subsurface gravity observations and density logs, Nevada Test Site 

and Hot Creek Valley, Nye County, Nevada: U.S. Geological 

Survey Professional Paper 700-B, p. B52-B62.

Hem, J.D., and Robertson, F.N., 1987, Hydrogeochemistry of 

ground-water recharge in alluvial aquifers, southern Arizona, 

in Let's get moving, Third symposium on artificial recharge of 

ground water in Arizona, May 20-21, 1987, Tempe, Arizona, 

Proceedings: Salt River Project, Phoenix, and University of Arizona, p. 30-51.

Holzer, T.L., 1980, Reconnaissance maps of earth fissures and land 

subsidence, Bowie and Willcox areas, Arizona: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1156, scale 

1:24,000, 2 sheets.

Keith, S.J., Paylore, Patricia, DeCook, K.J., and Wilson, L.G., 

1982, Bibliography on ground-water recharge in arid and 

semiarid areas: Tucson, University of Arizona, Water Resources Research Center report, 149 p.

Kisser, K.G., and Haimson, J.S., 1981, Estimations of aquifer characteristics using drillers' logs: Arizona and Nevada Academy of 

Sciences, Proceedings of 1981 meeting of American Water Resources Association and Hydrology Section, May 1-2, 1981, 

v. 11, p. 112-116.

Kister, L.R., 1973, Quality of ground water in the Lower Colorado 

River region, Arizona, Nevada, New Mexico, and Utah: U.S. 

Geological Survey Hydrologic Investigations Atlas HA-478, 

scale 1:1,000,000, 2 sheets.

Konieczki, A.D., and English, C.S., 1979, Maps showing ground-water 

conditions in the lower Santa Cruz area, Final, Pima, and Maricopa Counties, Arizona 1977: U.S. Geological Survey WaterResources Investigations Report 79-56, scale 1:125,000, 4 sheets.

Laney, R.L., 1979, Geohydrologic reconnaissance of Lake Mead recreation area Temple Bar to Grand Wash Cliffs, Arizona: U.S. 

Geological Survey Open-File Report 79-688, 71 p.

Laney, R.L., Raymond, R.H., and Winikka, C.C., 1978, Maps showing water-level declines, land subsidence, and earth fissures in 

south-central Arizona: U.S. Geological Survey Water-Resources 

Investigations Report 78-83, 2 sheets.

Langer, W.H., Mulvihill, D.A., and Anderson, T.W, 1984, Maps showing ground-water levels, springs, and depth to ground water, 

Basin and Range Province, Arizona: U.S. Geological Survey 

Water-Resources Investigations Report 83-4114-B, 2 sheets.

Leake, S.A., and Prudic, D.E., 1988, Documentation of a computer 

program to simulate aquifer-system compaction using the 

modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 88-482, 80 p.

Leonard, R.J., 1929, An earth fissure in southern Arizona: Journal 

of Geology, v. 37, no. 8, p. 765-774.

Lister, R.H., and Lister, F.C., 1983, Those who came before: Tucson, University of Arizona Press, 184 p.

Mattick, R.E., Olmsted, F.H., and Zohdy, A.A.R., 1973, Geophysical 

studies in the Yuma area, Arizona and California: U.S. Geological Survey Professional Paper 726-D, 36 p.
A32 REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

Meinzer, O.E., and Kelton, F.C., 1913, Geology and water resources 

of Sulphur Spring Valley, Arizona, with a section on Agriculture by R.H. Forbes: U.S. Geological Survey Water-Supply 

Paper 320, 231 p.

Oppenheimer, J.M., and Sumner, J.S., 1980, Regional geophysics 

assessment of southwest alluvial basins: University of Arizona, final report for U.S. Geological Survey contract 14-08- 

0001-18228, 50 p.

 1981, Gravity modeling of the basins in the Basin and 

Range Province, Arizona: Arizona Geological Society Digest, 

v. 13, p. 111-115.

Peirce, H.W., 1974, Thick evaporite deposits in the Basin and 

Range Province, Arizona, in Fourth Symposium on Salt: Cleveland, Ohio, Northern Ohio Geological Society, p. 47-55.

Poland, J.F., 1981, Subsidence in United States due to groundwater withdrawal: American Society of Civil Engineers, Journal of the Irrigation and Drainage Division, v. 107, no. IR2, 

p. 115-135.

Poland, J.F., Lofgren, B.E., and Riley, F.S., 1972, Glossary of selected terms useful in studies of the mechanics of aquifer systems 

and land subsidence due to fluid withdrawal: U.S. Geological 

Survey Water-Supply Paper 2025, 9 p.

Pool, D.R., 1984, Aquifer geology of alluvial basins of Arizona, in 

Replogle, J.A., and Renard, KG., eds., Water today and tomorrow: Specialty Conference, Irrigation and Drainage Division of 

the American Society of Civil Engineers, Flagstaff, Arizona, 

July 24-26, 1984, Proceedings, p. 683-690.

 1986, Aquifer geology of alluvial basins of Arizona, in Anderson, T.W., and Johnson, A.I., eds., Regional aquifer systems of 

the United States, Southwest alluvial basins of Arizona: American Water Resources Association Monograph Series 7, p. 25- 

36. 

-1987, Geohydrology of McMullen Valley, west-central Arizona: U.S. Geological Survey Water-Resources Investigations Report 87-4041, 51 p.

Robertson, F.N., 1984a, Isotopic content of ground water along the 

Colorado River in Arizona and California [abs.]: Geological Society of America Abstracts with Programs, 97th annual meeting, v. 16, no. 6, p. 636.

 1984b, Trace elements in ground water in southern Arizona 

[abs.], in Replogle, J.A., and Renard, K.G., eds., Water today 

and tomorrow: Specialty Conference, Irrigation and Drainage 

Division of the American Society of Civil Engineers, Flagstaff, 

Arizona, July 24-26, 1984, Proceedings, p. 674.

 1984c, Solubility controls of fluorine, barium, and chromium 

in ground water in alluvial basins of Arizona, in Hitchon, 

Brian, and Wallick, E.I., eds., Practical applications of ground 

water geochemistry: National Water Well Association, Proceedings, First Canadian-American Conference on Hydrogeology, 

June 22-26, 1984, Banff, Alberta, Canada, p. 96-102.

 1986a, Occurrence and solubility controls of trace elements 

in ground water in alluvial basins of Arizona, in Anderson, 

T.W., and Johnson, A.I., eds., Regional aquifer systems of the 

United States, Southwest alluvial basins of Arizona: American 

Water Resources Association Monograph Series 7, p. 69-80.

 1986b, Origin and chemistry of ground water along the Colorado River in Arizona, Nevada, and California [abs.]: Journal 

of the Arizona-Nevada Academy of Science, v. 21, proceedings 

supplement, p. 56.

 1989a, Ground-water geochemistry and information transfer 

in alluvial basins in Arizona: 28th International Geological 

Congress, July 9-19, 1989, Abstracts, v. 2, p. 709-710.

 1989b, Arsenic in ground water under oxidizing conditions, 

southwest United States: Environmental Geochemistry and 

Health, v. 11, no. 3/4, December 1989, p. 171-185.

 1989c, Prediction of water quality through geochemical modeling in undeveloped areas in the desert Southwest [abs.], in 

U.S. Geological Survey Second National Symposium on Water 

Quality, Abstracts of the Technical Sessions, Orlando, Florida, 

November 12-17, 1989: U.S. Geological Survey Open-File Report 89-409, p. 77-78.

 1990, Trace constituents in ground water in alluvial basins 

in Arizona source, controls, and prediction [abs.], in Programme of the Eighth Annual European Meeting, Society for 

Environmental Geochemistry and Health: Reading, England, 

Reading University, April 1 4, 1990. 

-1991, Geochemistry of ground water in alluvial basins in

Arizona and adjacent States: U.S. Geological Survey Professional Paper 1406-C, 90 p.

Robertson, F.N., and Garrett, W.B., 1986, Distribution of fluoride 

in the ground water in alluvial basins of Arizona and adjacent parts of California, Nevada, and New Mexico: U.S. Geological Survey Hydrologic Investigations Atlas HA-665, scale 

1:500,000, 3 sheets.

Scarborough, R.B., and Peirce, H.W, 1978, Late Cenozoic basins 

of Arizona, in Callender, J.F., Wilt, J.C., and demons, R.E., 

eds., The Land of Cochise: New Mexico Geological Society 

Guidebook, 29th Field Conference, p. 253-259.

Schmidt, K.D., 1980, Ground-water quality impact determined 

from well sampling, in Proceedings of the Deep Percolation 

Symposium, May 1-2, 1980: Arizona Department of Water 

Resources Report 1, p. 74-84.

Schumann, H.H., and Genualdi, R.B., 1986, Land subsidence, 

earth fissures, and water-level change in southern Arizona: 

Arizona Bureau of Geology and Mineral Technology, Map 23, 

scale 1:1,000,000, 1 sheet.

Schumann, H.H., Laney, R.L., and Gripe, L.S., 1986, Land subsidence and earth fissures caused by ground-water depletion in 

southern Arizona, in Anderson, T.W, and Johnson, A.I., eds., 

Regional aquifer systems of the United States, Southwest alluvial basins of Arizona: American Water Resources Association Monograph Series 7, p. 81-91.

Schumann, H.H., and Poland, J.F., 1970, Land subsidence, earth 

fissures, and ground-water withdrawal in south-central Arizona, U.S.A., in Land subsidence: Tokyo, International Association of Scientific Hydrology Publication 88, v. 1, p. 295- 

302.

Schumann, H.H., Tosline, D.J., and Wrege, B.M., 1984, Occurrence and prediction of earth-fissure hazards caused by 

ground-water depletion in south-central Arizona, U.S.A. 

[abs.], in Replogle, J.A., and Renard, KG., eds., Water today 

and tomorrow: Specialty Conference, Irrigation and Drainage 

Division of the American Society of Civil Engineers, Flagstaff, Arizona, July 24-26, 1984, Proceedings, p. 673.

Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, 

W.A., and Raymond, R.H., 1980, K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas: Arizona Geological Society Digest, v. 12, p. 201-260.

Small, G.G., 1983, Ground-water quality impacts of cascading 

water in the Salt River Project area, in Proceedings of the 

Deep Percolation Symposium, October 26, 1982: Arizona Department of Water Resources Report 4, p. 41-47.

Thompson, T.H., Nuter, Janet, and Anderson, T.W., 1984, Maps 

showing distribution of dissolved solids and dominant chemical type in ground water, Basin and Range Province, Arizona: U.S. Geological Survey Water-Resources Investigations 

Report 83-4114-C, 4 sheets.

Trescott, PC., 1975, Documentation of finite-difference model for 

simulation of three-dimensional ground-water flow: U.S. Geological Survey Open-File Report 75-438, 32 p.
SUMMARY OF THE SOUTHWEST ALLUVIAL BASINS, SOUTH-CENTRAL ARIZONA AND PARTS OF ADJACENT STATES A33

Trescott, PC., and Larson, S.R, 1976, Documentation of finite-difference model for simulation of three-dimensional groundwater flow: U.S. Geological Survey Open-File Report 76-591, 

21 p. [Supplement to Open-File Report 75-438.]

Tucci, Patrick, 1982, Use of three-dimensional model for the analysis of the ground-water flow system in Parker Valley, Arizona 

and California: U.S. Geological Survey Open-File Report 82- 

1006, 54 p.

 1984, Surface resistivity studies for water-resources investigations, near Tucson, Arizona: Proceedings, National Water 

Well Association-U.S. Environmental Protection Agency Conference on surface and borehole geophysical methods in 

ground-water investigations, San Antonio, Texas, February 6- 

9, 1984, p. 92-106. 

-1989, Geophysical methods for water-resources studies in

ration Geophysicists, Expanded Abstracts with Biographies, 

1983 Technical Programs, p. 28-31. 

-1984, Density of basin-fill deposits calculated from boreholesouthern and central Arizona, in Proceedings of the Symposium on the Application of Geophysics to Engineering and 

Environmental Problems: Denver, Colorado, Society of Engineering and Mineral Exploration Geophysicists, p. 368-383.

Tucci, Patrick, and Pool, D.R., 1982, Borehole gravity surveys in 

basin-fill deposits of central and southern Arizona: U.S. Geological Survey Open-File Report 82-473, 23 p.

1986, Use of geophysics studies in the alluvial basins of Arizona, in 

Anderson, T.W, and Johnson, A.I., eds., Regional aquifer systems of the United States, Southwest alluvial basins of Arizona: American Water Resources Association Monograph 

Series 7, p. 37-56.

Tucci, Patrick, Schmoker, J.W, and Robbins, S.L., 1983, Density of 

basin-fill deposits calculated from borehole-gravity data in four 

basins in central and southern Arizona [abs.]: Society of Explogravity data in four basins in central and southern Arizona 

tabs.]: Geophysics, v. 49, no. 5, May 1984, p. 595.

U.S. Environmental Protection Agency, 1977, National interim primary drinking-water regulations: U.S. Environmental Protection Agency Report EPA-470/9-76-003, 159 p.

U.S. Geological Survey, 1982, Annual summary of ground-water 

conditions in Arizona, spring 1980 to spring 1981: U.S. Geological Survey Open-File Report 82-368, scale 1:1,000,000, 

2 sheets.

Valley National Bank, 1983, Arizona statistical review: Phoenix, 

Valley National Bank of Arizona, Economic Research Department, 39th annual edition, 76 p.

White, N.D., and Anderson, T.W, 1985, Arizona ground-water 

resources, in National water summary, 1984 Hydrologic events, 

selected water-quality trends, and ground-water resources: U.S. 

Geological Survey Water-Supply Paper 2275, p. 135-140.

Wilson, E.D., Moore, R.T., and Cooper, J.R., 1969, Geological map of 

Arizona: Arizona Bureau of Mines map, scale 1:500,000, 1 sheet.

Wilson, L.G., DeCook, K.J., and Neuman, S.P, 1980, Regional recharge research for southwest alluvial basins: Tucson, University 

of Arizona, Water Resources Research Center report, 389 p.

Winikka, C.C., and Wold, P.O., 1977, Land subsidence in central 

Arizona: International Association of Scientific Hydrology Publication 121, p. 95-103.

Winograd, I.J., and Robertson, F.N., 1982, Deep oxygenated ground 

water Anomaly or common occurrence?: Science, v. 216, 

p. 1227-1230.

U.S. GOVERNMENT PRINTING OFFICE: 1995 - 673-046 / 20008 REGION NO. 8

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

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SOU REGIONAL AQUIFER-SYSTEM ANALYSIS SOUTHWEST ALLUVIAL BASINS, AkIZONA AND ADJACENT SI AlfcS U.S. Geologi