Data Series 301
U.S. GEOLOGICAL SURVEY
Data Series 301
The Southern Sierra study unit (SOSA) covers approximately 1,800 square miles in Kern and Tulare Counties, California, at the southern end of the Sierra Nevada hydrogeologic province (fig. 1). The study unit area is defined by the watersheds of Tejon Creek, Tehachapi Creek, the middle and upper Kern River, and the East Tehachapi closed drainage basin. SOSA includes 6 small ground-water basins, as defined by the California Department of Water Resources (fig. 2) (California Department of Water Resources, 2003). The wells sampled in the southern part of SOSA are in or nearby the Cummings Valley, Brite Valley, Tehachapi Valley West, and Tehachapi Valley East ground-water basins. The wells sampled in the northern part of SOSA are in or near the Kern River Valley ground-water basin.
SOSA has approximately 8,000 ft of topographic relief. The Cummings and Tehachapi basins are relatively flat with an altitude of approximately 4,000 ft. The Tehachapi Mountains rise to 7,700 ft south of the Cummings basin, and to over 8,000 ft south of the Tehachapi basin. Water level in Lake Isabella is 2,600 ft, and the peaks of the Sierra Nevada rise up to 10,000 ft as near as 40 mi north of the lake. The climate in SOSA is characterized by hot, dry summers and cold, wet winters, although the temperatures and precipitation are strongly controlled by elevation. Precipitation at Lake Isabella Dam (2,635 ft) and in the city of Tehachapi (4,017 ft) averages 11 inches per year, with 80 to 90 percent falling between November and April (California Department of Water Resources, 2007). Winter precipitation falls mostly as snow at elevations above 5,000 ft.
The Cummings, Brite, Tehachapi West, and Tehachapi East basins are all bounded by the Tehachapi Mountains to the south and the Sierra Nevada to the north, and are separated by north to northwest trending low bedrock ridges or alluvial divides (California Department of Water Resources, 2006a, b, c, d). The primary water-bearing units are Pleistocene to Recent alluvial fans around the margins of the basins deposited by creeks draining the Tehachapi Mountains and Sierra Nevada, and floodplain deposits in the centers of the basins (Dibblee and Warne, 1970; Dibblee and Louke, 1970; Michael and McCann, 1962). The sediments are arkosic cobbles, gravels, sands, silts, and clays, with the coarser materials in the alluvial fans and the finer sediments in the floodplains. In the Tehachapi East basin, water is also found in Miocene to Pliocene sedimentary and volcanic units.
Cummings, Brite, Tehachapi West, and Tehachapi East are all adjudicated basins and are managed collectively by the Tehachapi-Cummings County Water District (TCCWD). Beginning in 1973, TCCWD has imported water from the State Water Project via a pipeline just upstream of the A.D. Edmiston pumping plant (Tehachapi-Cummings County Water District, 2004a,b). The system for distributing this water to the TCCWD area has expanded over the years so that Cummings, Brite, Tehachapi, and Bear Valleys are all now supplied with imported water.
The 124-square-mile Kern River basin has an irregular shape that follows the dendritic drainage pattern of the Kern River and its tributary creeks. The western arm of the basin lies in the trace of the Kern Canyon fault. The primary water-bearing units are the Pleistocene to Recent fluvial deposits in the channel of the Kern River and around Lake Isabella (California Department of Water Resources, 2006e).
Examination of driller’s logs for wells in areas of SOSA outside of the alluvial basins showed that the wells were completed in fractured hard-rock. The Mesozoic granitic rocks and Paleozoic to Mesozoic metamorphic rocks of the Sierra Nevada range are apparently locally water-bearing. Little is known about these aquifers.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/301
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