Hydrologic and Legal Issues of the Upper San Pedro River Basin, Arizona, cont.

(L. Lacher, 1994)

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Geology and Geography

The San Pedro River Basin lies within the Basin and Range Physiographic Province in the southwest United States. The Basin and Range Province is characterized by elongated north-south tending mountain chains separated by alluvium filled river valleys. The Upper San Pedro Basin (USPB) extends from about 23 miles south of the International Boundary with Mexico to about 74 miles north of the Boundary near Benson, Arizona (Figure 1). The division between the upper and lower San Pedro River Basins falls in an area known as "The Narrows," which consists of a bedrock constriction in the valley 12 miles north of Benson. The Upper Basin contains Benson and Sierra Vista subwatersheds and drains an area of about 2500 square miles at the United States Geological Survey (USGS) streamgage near Benson. Approximately 700 square miles (28%) of this drainage area lie in Mexico, with the remaining 1800 square miles contained in Arizona. The San Pedro River has a length of 90 miles from its headwaters in Mexico to the USGS streamgaging station at Benson (Putman, et al, 1988). Figure 2 shows the distribution of mountains bounding the San Pedro Basin. The west boundary of the San Pedro Basin is marked by the Huachuca, Mustang, and Whetstone Mountains, as well as the Canelo Hills. The Tombstone Hills and Mule Mountains form the east border of the basin (Freethey, 1982).

The basin fill slopes gently from south to north, resulting in a northward river flow direction. The floodplain gradient averages about 12 to 15 feet per mile (Freethey, 1982). Mountain altitudes range from 4,400 to 9,500 feet (ft.) above mean sea level (MSL) (Freethey, 1982), while valley floor elevations vary from about 4,800 feet near the headwaters in Mexico to 3,300 feet above MSL at the Narrows north of Benson (Putman, et al, 1988).

The mountains bordering the basin are composed of Precambrian to Tertiary crystalline granitic and metamorphic, volcanic, and consolidated sedimentary rock formations. These units form the structural basement of the basin and their upper surface may be as deep as 5,000 feet (Oppenheimer and Sumner, 1980). These units are generally of low permeability and porosity except where highly fractured. Wells in these units may yield a few gallons per minute (gpm), enough for domestic and livestock purposes (Vionnet and Maddock, 1994). Some springs in the Huachuca Mountains yield as much as several hundred gpm (Brown, et al, 1966).

The Pantano formation, a Tertiary conglomerate, overlies the bedrock units. This unit is well cemented and has low permeability and porosity. Wells in this unit yield small to moderate amounts of water up to several hundred gpm in some faulted areas (Roeske and Werrell, 1974). The Quaternary Valley Fill deposits unconformably overly the Pantano formation and make up the main water bearing unit in the basin. The lower basin fill unit consists of sandstone, lenticular gravel, and siltstone and has a maximum thickness of about 1000 feet in the center of the basin (Roeske and Werrell, 1974). The upper unit contains poorly cemented clay, gravel, sand and silt, fining toward the valley center (U.S. Army Corps of Engin., 1974). This unit ranges in thickness from about 620 feet at Fort Huachuca well field to a few feet thick along the San Pedro River

(U.S. Army Corps of Engin., 1974). In the well-bedded silty sand and clays of the central valley, groundwater is under artesian pressure. Otherwise, groundwater occurs mainly under unconfined conditions (Vionnet and Maddock, 1992). The alluvial sediments of the river flood plain are generally coarse grained, poorly cemented, and have hydraulic conductivities 2 to 10 times higher than basin fill deposits (Freethey, 1982).

Surface Water

The San Pedro River originates about 30 miles south of the International Border, north of Cananea, Sonora, Mexico. The river enters the United States near Palominas, Arizona and flows northward toward its confluence with the Gila River. Most of its tributaries are ephemeral, flowing only in response to rainfall or snowmelt events. The San Pedro River is perennial (flowing continuously all year) for approximately 36 of the 62 river miles in the United States portion of the Upper San Pedro River Basin. The perennial reach typically extends from Hereford, Arizona to a diversion dam about 5 miles south of St. David, Arizona (ADWR, 1990). During the early summer months, the only section of river that sustains base flow is that between Hereford and Charleston. The floodplain aquifer recovers during the winter, with streamflow returning over most of the river reach in November or December, and remaining until March or April (ADWR, 1990). Outside of this perennial reach, the San Pedro flows intermittently. Most of the perennial reach of the San Pedro River falls inside the San Pedro Riparian National Conservation Area (SPRNCA), a federal reserve managed by the United States Bureau of Land Management (BLM).

The Babocomari River is the major tributary to the San Pedro River within the Upper portion of the basin. The Babocomari flows perennially over two reaches for a total of twelve miles (ADWR, 1990). The Babocomari drains the Mustang Mountains, Canelo Hills, and the north end of the Huachuca Mountains. The Babocomari enters the San Pedro River just south of Fairbank, Arizona. Perennial flows have also been identified on some of the other San Pedro River tributaries in the Upper San Pedro Basin. Miller and Ramsey Canyons in the Huachuca Mountains sustain perennial flows, and the Canelo Hills region (southwest of Elgin) contains many swampy areas known as "cienegas." These surface flows are generally produced by geologic constrictions in the subsurface that force water to the surface (ADWR, 1990). Annual average tributary runoff to the San Pedro River is estimated at 35,800 acre-feet (ac-ft) between the Palominas and the Narrows streamgages (Putman, et al, 1988).

Seepage studies by the USGS in 1969 and 1970 indicate that the San Pedro loses streamflow (stream water is lost to infiltration through the riverbed) from the International boundary to Palominas, Arizona at a rate of 1.7 cubic feet per second (cfs). The river then gains flow (stream flow is augmented by flow from the groundwater system) from Palominas to Charleston, Arizona (+8.5 cfs), and loses again from Charleston to the mouth of the Babocomari River near Fairbank (-0.4 cfs) (Freethey, 1982).

Like many rivers in the southwestern United States, the San Pedro River has two major flow components: runoff and baseflow. Runoff occurs after precipitation events or as result of snowmelt, and lasts a few days until all flow is either lost to outflow from the basin or to bank storage along river. The highest annual flows in San Pedro River and its tributaries occur between July and September, and are typically of short duration. Longer duration, lower-discharge peak flows occur in winter. Baseflow results from the discharge of groundwater to the stream and sustains streamflow in dry seasons. Perennial flow in the Hereford to Fairbank reach is maintained by baseflow produced by a bedrock constriction near Charleston. This constriction forces groundwater in the alluvial aquifer to discharge into the San Pedro River (Putman, et al, 1988).

Annual runoff volumes show no trend either up or down for the period 1931-1985 (Jackson, et al, 1987), and give no indication of the influence, in any, of recent groundwater pumping on streamflows (Vionnet and Maddock, 1992). Intermittent flows in other reaches of the San Pedro River result from the timing of water uses (by crops and riparian vegetation) along the stream and from the climatic regime (Putman, et al, 1988). Some evidence indicates that these intermittent reaches may have been perennial prior to cultural development (Brown, et al, 1981). Annual flows in the San Pedro River are erratic from year to year, with lower than average flows predominating. Although flow at any one gage varies from year to year, all stream gages on the Upper San Pedro show that flows tend to rise and fall together (Putman, et al, 1988). Table 1 shows average annual discharge and runoff for streamflow gaging stations in the Upper San Pedro Basin for the common period of record 1968-1976 (after table 2-3, ADWR, 1990)

Precipitation and Climate

Average annual precipitation in the Upper San Pedro Basin varies from about 10 to 12 inches along the river up to greater than 30 inches in Huachuca Mountains (ADWR, 1990). Precipitation in southeastern Arizona is bimodally distributed with about 50 to 60% of the annual total falling in the summer "monsoon" season, and 21% to 35% occurring in the winter months. Spring and fall are typically dry. Winter storms originate mainly from mid-latitude Pacific (cyclonic) fronts, producing several days of gentle rain, moderate winds, and occasional snow (Sellers and Hill, 1974). Summer storms derive moisture from the Gulf of Mexico and are orographic, convection type thunderstorms of high intensity and short duration. Maximum "monsoonal" precipitation falls on the southeast (windward) side of the mountains (Putman, et al, 1988).

The climate of the Upper San Pedro River Basin is arid to semi-arid. Unlike nearby desert areas such as the Tucson Basin, the Upper San Pedro River Basin does not suffer extreme desert conditions because of its higher elevation. Summer temperatures rarely reach 100E Fahrenheit in the higher elevations, but frequently exceed 100EF in some lower elevations of the basin. Table 2 lists basic climatic statistics for some of the population centers in the Upper San Pedro Basin.

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