skip to the content of this page Arizona's Official Website Arizona Department of Water Resources
Arizona Department of Water Resources Arizona's Official Web Site
Securing Arizona's Water Future

Lower Colorado River Hydrology - Groundwater (West Basins)

Colorado River, Lower Gila Basin

Colorado River in the Lower Gila Basin

West Basins

West Basins include Butler Valley, Gila Bend, Harquahala, Lower Gila, McMullen Valley, Ranegras Plain, San Simon Wash, Tiger Wash and Western Mexican Drainage basins.  Groundwater inflows and outflows are relatively small in these basins and there are no perennial streams.  Groundwater inflows consist of minor amounts of mountain front recharge and stream infiltration.  The basins are contain a relatively thin, heterogeneous layer of upper basin fill underlain by lower basin fill.  The lower basin fill consists of a unit of primarily fine-grained material underlain by a medium to coarse grained unit.  Pre-Basin and Range sediments underlie the basin fill.  Stream alluvium deposits occur along the Gila River and elsewhere and may be locally productive water-bearing sediments (Anderson and others, 1992).

Butler Valley Basin

Butler Valley Basin contains basin-fill deposits that make up the principal aquifer.  These deposits range from about 500 feet in the southwest to nearly 1,500 feet thick in the central portion of the basin.  The valley is bordered by mountains and some groundwater may be found along the basin margins in thin alluvium and in volcanic, granitic, metamorphic and sedimentary rocks.  A 1½-mile wide area bordered by mountains where Cunningham Wash exits the basin is known as the Narrows. Groundwater is found under confined conditions northeast of the Narrows in T7N, R15W and confined conditions may occur in other areas due to the presence of clay layers. Groundwater flow is generally from northeast to southwest (Oram, 1987).

Groundwater recharge is approximately 1,000 AFA or less.  Groundwater storage estimates range widely from 2.0 to 20 maf (Table 7.1-3).  The median well yield reported for 17 large diameter (>10 in.) wells is 2,200 gpm.  Water levels declined in most wells measured between 1990-’91 and 2003-’04, with the recent water level measurements generally ranging from 100 to 500 feet bls (Figure 7.1-5).

Groundwater quality is generally good with locally elevated fluoride and arsenic concentrations measured in wells located in the western part of the basin (Figure 7.1-8)

Gila Bend Basin

Basin-fill material is the principal aquifer in the Gila Bend Basin.  Groundwater generally occurs under unconfined conditions, but there are several areas where fine-grained layers in the alluvium create either overlying perched water-table conditions as a result of percolation of irrigation water or underlying confined conditions.  Confined conditions occur in the upper basin fill immediately upstream from Painted Rock Dam (Rascona, 1996).

West of Gila Bend, significant clay layers ranging from 150 to 500 feet thick are found at various depths and depth to water increases southward.  North of Gila Bend, unconfined groundwater occurs primarily in the sands and gravels of the basin fill and may also occur in interbedded volcanics. The Sil Murk Formation is one of the principal water-bearing formations in the lower basin fill in this area. It is comprised of pebble to boulder-sized conglomerates with thin interbedded volcanics near the top. (Rascona, 1996)

Gillespie Dam

Gilespie Dam in the Gila Bend Basin

In the area north of Gila Bend, groundwater flow direction is generally from the Gila Bend Mountains east to the Gila River. In the center of the basin, groundwater flow is toward the southwest (see Figure 7.2-6).

Groundwater is recharged primarily from infiltration of surface flows from the Gila River and its tributaries, and when river water is impounded behind Painted Rock Dam. Some recharge also occurs from infiltration of irrigation water and underflow from the Hassayampa sub-basin of the Phoenix AMA (<1,000 AFA) (Rascona, 1996).  Annual recharge estimates range from 10,000 to 37,000 AFA.  Groundwater storage estimates range widely from 17 to 61 maf.  The median well yield reported for 242 large diameter (>10 in.) wells is high with 2,700 gpm (Table 7.2-6).

Water levels in wells measured in 2003-‘04 ranged from 34 feet in a well along the mountain front to almost 640 feet east of Gila Bend. Groundwater pumpage historically caused several cones of depression to form, with the largest cone north of Gila Bend and parallel to the Gila River.  As shown in Figure 7.2-6 water level declines are still significant (>30 feet) in wells in this area and almost all wells measured between 1990-’91 and 2003-’04 showed some decline.

Groundwater quality is generally poor across the basin with several measurements of arsenic and fluoride concentrations meeting or exceeding drinking water standards. High concentrations of TDS and nitrate have also been detected (see Table 7.2-7).

Big Horn Mountains, Harquahala Basin

Big Horn Mountains, Harquahala Basin

Harquahala Basin

Groundwater in the Harquahala Basin is found primarily in basin-fill material composed of heterogeneous deposits of clay, silt, sand and gravel.  The basin fill may be as much as 5,000 feet thick near Centennial.  Groundwater is generally unconfined, although clay layers can cause locally semi-confined to confined conditions.  Clay layers also cause perched water-table conditions in the east-central and southeastern parts of the basin from percolation of irrigation water.  In the southeastern part of the basin the basin fill consists of coarse deposits of sand and gravel. North of T1S, fine-grained beds primarily composed of clay overly the coarse deposits.  Wells in this area penetrate the fine-grained sequence and withdraw water from the underlying coarse-grained sequence. The fine-grained beds become thicker towards the northwest and grade into an alternating sequence of fine-grained and coarse-grained layers that overlie a conglomerate that begins at a depth of 800 to 850 feet bls. (Hedley, 1990)  Reportedly, the best well yields occur from this alternating sequence in the west-central part of the basin.

Prior to the 1950s groundwater moved from northwest to southeast and exited where Centennial Wash leaves the basin. As shown in Figure 7.3-5, groundwater flow in the south central part of the basin has been impacted by agricultural pumpage that caused severe overdraft from the 1950s through the mid 1980s, resulting in large water level declines and formation of a cone of depression.

Groundwater recharge is negligible, coming primarily from infiltration of runoff in Centennial Wash.  There may also be underflow from McMullen Valley Basin to the north.  Seepage and infiltration of water from the Central Arizona Project (CAP) canal, which runs west to east across the southern part of the basin, may be another source of recharge. Estimated annual recharge was less than 1,200 AFA. Groundwater storage estimates range from 13 to 27 maf.  The median well yield reported for 157 large diameter (>10 in.) wells is 1,620 gpm (Table 7.3-5).

Introduction of CAP water in the late 1980s replaced a significant volume of groundwater pumping, allowing groundwater levels to rise by more than 30 feet in a number of wells in the south central part of the basin.  Storage of CAP water at the Vidler Recharge facility has also caused local groundwater levels to rise. Elsewhere, water levels have generally declined (see Figure 7.3-5).  The Harquahala Basin was designated an INA in 1984 pursuant to A.R.S. § 45-432 to prevent new lands from being brought into agricultural production.  However, under A.R.S. § 45-555 groundwater may be withdrawn and transported from the basin to an initial active management area (such as the adjacent Phoenix AMA) under specific circumstances including a provision that groundwater levels not decline by an average of more than ten feet per year.

Groundwater quality is generally suitable for irrigation purposes, but elevated TDS, fluoride, arsenic and other constituent concentrations in many wells require treatment to meet drinking water standards (see Table 7.3-6).

Lower Gila Basin

The Lower Gila Basin is composed of the Wellton-Mohawk sub-basin, the Dendora Valley sub-basin in the northeast and the Childs Valley sub-basin in the southeast (Figure 7.4-6). Groundwater occurs in both recent stream alluvium and basin fill.  The stream alluvium consists of sand, gravel and boulders in the larger washes and the floodplain of the Gila River.  The thickness of the stream alluvium ranges from 10 feet in smaller washes to 110 feet in the Gila River floodplain.  The basin fill consists of three units.  The upper sandy unit is composed of sand and gravel with some silt and clay layers. This unit is typically 200 to 380 feet thick.  The middle fine-grained unit contains primarily silts and clays with occasional thin sand and gravel beds. The middle unit ranges from 250 to 750 feet thick.  The lower coarse-grained unit is composed of coarse sand and gravel and contains some well-cemented zones.  The thickness of this unit is variable. Groundwater development in the eastern part of the Lower Gila Basin is in the broad alluvial plains that border the Gila River, where the main aquifer is the upper sandy unit in the basin fill.  Groundwater is primarily unconfined.

Mohawk Mountains, Lower Gila Basin

Mohawk Mountains, Lower Gila Basin

Prior to development, groundwater flow was from north and southeast toward the Gila River and then downstream to the southwest.  Groundwater flow has been impacted by irrigation pumpage at some locations in the basin, where cones of depression exist (see Figure 7.4-6). Historically, cones of depression occurred in irrigated areas north of Hyder, east of Dateland and in the Palomas Plain west of Hyder.  Infiltration of irrigation water in the western part of the basin has created groundwater mounds in the floodplain aquifer that also affect groundwater flow.

Groundwater recharge is primarily from infiltration of runoff in washes and the Gila River floodplain.  Underflow from the Painted Rock Dam on the eastern basin boundary and releases from the dam during floods also contributes to groundwater recharge. Water releases from Painted Rock Dam in 1975 resulted in an estimated 59,500 acre-feet of recharge.  In the far western part of the basin, infiltration of excess irrigation water is the largest source of groundwater recharge.  Estimates of natural groundwater recharge ranging from 9,000 to 88,000 AFA.

There is a significant volume of groundwater in storage with estimates ranging from 100 to 246 maf. The median well yield reported for 597 large diameter (>10 in.) wells is 1,600 gpm (Table 7.4-6).  Well yields exceeding 2,000 gpm are commonly found near the Gila River, southeast of Dateland and north of Hyder.

Groundwater levels in the Gila River floodplain in the western part of the basin historically ranged from 10 to 20 feet bls and the streambed alluvium was the primary source of groundwater. As irrigation activity increased in the 1930s, groundwater levels declined and salinity increased.  To provide a dependable water supply for irrigation, Colorado River water was brought to the area in 1952 and groundwater pumping for irrigation ceased.  Infiltration of excess irrigation water to the stream alluvium aquifer raised water levels, necessitating the need for a system of drainage wells to maintain groundwater levels below crop root zones and canals to transport the drainage water out of the basin.

Historic groundwater level declines were as much as 15 feet per year in irrigated areas north and west of Hyder and east of Dateland.  Few water level change measurements are available for the period 1990-’91 to 2004-’05 but several measured wells in the western part of the basin show relatively stable water level conditions (see Figure 7.4-6). 

Groundwater quality varies in the eastern part of the basin with elevated fluoride concentrations measured in a number of wells.  In the western part of the basin, the quality of groundwater in the Gila River floodplain is unsuitable for most uses, with elevated TDS concentrations common as well as fluoride and arsenic.


water drop  Continue to Section 7.0.2 Hydrology - Groundwater (West Basins - Continued) or skip to 7.0.2 Hydrology - Surface Water


Arizona Water Atlas Home

Lower Colorado River Planning Area Home

Download pdf of Lower Colorado River Planning Area Overview

Colorado River
Lower Colorado River Planning Area

Volume 7

Parker Dam
Agriculture in Yuma Basin