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Hydrology of the Upper San Pedro Basin

Groundwater Hydrology

Western Portion

On the western side of the planning area are a group of basins that are tributary to the San Pedro and Gila rivers; Aravaipa Canyon, Donnelly Wash, Lower San Pedro and Upper San Pedro. Groundwater is found in stream alluvium and basin fill sediments in these basins. 

Upper San Pedro Basin

The Upper San Pedro Basin consists of the northwest trending San Pedro River Valley and surrounding mountains that range from 5,000 to almost 10,000 feet in elevation. The basin contains two sub-basins: the Sierra Vista and the small Allen Flat sub-basin. Basin fill is the principal aquifer although the stream alluvium is also utilized.  Groundwater in the basin fill aquifer is found in both unconfined and confined conditions. Artesian conditions exist near Palominas, Hereford, and more extensively near Benson and Saint David. These conditions supported modest groundwater discharges for irrigation use primarily in the Benson-Pomerene area. An interesting feature is a limestone aquifer in the Whetstone Mountains that contains a “live” or wet cave, Kartchner Caverns, a state park.  The water level in the cavern is about 700 feet higher than that of the underlying alluvial aquifer (ADWR, 2005a). 

Groundwater flow direction is from the mountain fronts toward the central valley and to the north. A cone of depression has formed in the Sierra Vista area that has altered flow direction (Figure 3.13-6). Groundwater recharge is approximately 35,700 AFA from the mountain fronts, underflow from Mexico and streambed infiltration. Two effluent recharge projects in the basin also recharge the aquifer. The most populous basin in the planning area, major discharge is from municipal and agricultural pumpage and from riparian evapotranspiration. (ADWR, 2005a)  The most recent estimate of groundwater in storage is 19.8 to 26.1 maf although estimates of up to 59 maf  exist (Table 3.13-5).

As shown in Figure 3.13-6, water levels declined in most wells measured in 1990-1991 and 2003-2004.  Additional data show annual declines of 0.9 to 2.9 feet in some wells in the Bisbee-Naco area and rises of up to 0.6 feet per year in the Pomerene area north of Benson (ADWR, 2005a).

Groundwater quality is generally suitable for most uses. Arsenic and fluoride were the water quality parameters that most frequently exceeded drinking water standards in wells sampled in the basin. Localized nitrate contamination near St. David is being remediated as part of the Superfund Program.

Click to view Table 3.13-5

Click to view Table 3.13-5 Groundwater Data for the

Upper San Pedro Basin

Click to view Figure 3.13-7

Click to view Figure 3.13-6 Upper San Pedro Basin

Groundwater Conditions

Figure 3.0-10

USGS 6-Digit Hydrologic Unit Code (HUC) Boundaries in the Southeastern Arizona Planning Area

San Pedro-Willcox Watershed

The Arizona portion of the San Pedro River Watershed is contained entirely within the planning area.  Approximately 696 square miles of the Watershed extends into Mexico. In Arizona, the Watershed includes all of the Aravaipa Canyon and Upper San Pedro basins, most of the Lower San Pedro and Willcox basins and relatively small portions of the Cienega Creek, Douglas and San Rafael basins.  A few tributaries to the San Pedro River begin on the southwest slopes of the Huachuca Mountains in the San Rafael Basin and drain into Mexico. (ADWR, 2005a) The San Pedro River enters the U.S. from Mexico near Palominas (see Figure 3.13-1) and flows north to its confluence with the Gila River. Major tributaries are the Babocomari River and Aravaipa Creek.

With the exception of Whitewater Draw in the extreme southern end of the basin that drains into the Douglas Basin, most of the surface water drainage in the Willcox Basin is to the Willcox Playa.  The playa occupies about 50 square miles in the center of the basin and is a remnant of Pleistocene-age Lake Cochise. (Oram, 1993)


Some stretches of the San Pedro River are perennial, although recent drought and delay of the summer monsoon has affected some previously perennial stretches for short periods of time, most notably at Charleston in the Upper San Pedro Basin. The Babocomari River, in the Upper San Pedro Basin, is perennial in its upper reach. Aravaipa Creek is perennial within Aravaipa Canyon above its confluence with the San Pedro River as are three of its tributaries in the Aravaipa Canyon Basin (see Figures 3.1-5 and 3.8-6).  Other perennial streams are found in the Lower San Pedro, Upper San Pedro and Willcox basins (Figures 3.8-6, 3.13-5 and 3.14-5).

There are 12 active streamgages in the watershed; two in the Lower San Pedro Basin and 10 in the Upper San Pedro Basin. The gage on the San Pedro River at Charleston has been in operation since 1904. The largest annual flow ever measured in the watershed, (152,798 acre-feet), was recorded at this gage in 1914.  More recently, in 1984, a maximum annual flow of 102,107 acre-feet was measured at the gage on the San Pedro River near Tombstone.  Median annual flow at these gages is 33,203 acre-feet and 29,654 acre-feet, respectively.

The only major springs in the watershed are found in the Lower San Pedro and Upper San Pedro basins. There are 14 major springs in the Lower San Pedro Basin. The largest, Cooks Lake Spring, had a discharge rate of 1,000 gpm when last measured in 1951.  Twelve major springs have been identified in the Upper San Pedro Basin. The largest is Garden Canyon No.1 with a discharge of 134 gpm measured in 1963. Most of the spring measurements in both basins date from before 1980 and may not be indicative of current conditions (see Tables 3.8-5 and 3.13-5).

Fifteen miles of the San Pedro River in the Lower San Pedro Basin from Aravaipa Creek to the Gila River are impaired due to elevated concentrations of E. coli and selenium (Table 3.8-7). In the Upper San Pedro Basin, water quality standards were exceeded in three reaches of the San Pedro River for a total of 53 miles. These reaches are impaired due to elevated levels of E. coli, nitrate and copper (Table 3.13-6).




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