Scientific Investigations Report 2005–5288
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5288
Carson Valley lies in the rainshadow of the Sierra Nevada, with precipitation at the town of Minden, averaging 8.4 in/yr for 1971–2000 (National Oceanic and Atmospheric Administration, 2002, p. 12). For the same period, precipitation averaged about 40 in/yr at the top of the Carson Range to the west, and precipitation averaged 15–18 in/yr near the top of the Pine Nut Mountains to the east (Maurer and Halford, 2004, p. 35). Precipitation over most of the northern and eastern sides of Carson Valley was estimated to average 10–15 in/yr for 1971–2000 (Maurer and Halford, 2004, p. 33–34). Precipitation data for the eastern side of Carson Valley, about 7 mi east of Minden, indicates annual precipitation ranged from about 16 in. in 1995 to about 5 in. from 1999 through 2002 (period of record 1991–2002; Fish Springs RAWS site, Western Region Climate Center, written commun., 2003). Monthly precipitation at that location ranges from a maximum of about 1 in. from December through March to a minimum of about 0.3 in. from July through September (period of record 1991–2002; Fish Springs RAWS site, Western Region Climate Center, written commun., 2003).
Since 1999, annual precipitation near Minden, which generally is representative of conditions throughout the subarea, has been less than average, with 2004 being the sixth consecutive year of average or below average precipitation. Annual precipitation was slightly less than average in 2001, but was considerably less than average from 2002 through 2004 (fig. 4).
The hydrology of Carson Valley is dominated by flow of the Carson River. Average annual inflow (1990–2002) from the East Fork Carson River near Gardnerville, Nevada, was 257,000 acre-ft and from the West Fork Carson River at Woodfords, California, was 75,150 acre-ft (Maurer and others, 2004, p. 14), for a total of about 332,000 acre-ft. Average annual outflow of the mainstem Carson River was 287,300 acre-ft for that period (Maurer and others, 2004, p. 14). Thirteen perennial streams drain the Carson Range and are tributary to the floor of Carson Valley (Maurer and others, 2004), whereas only two perennial streams, Buckeye and Pine Nut Creeks, drain the Pine Nut Mountains (fig. 2).
Infiltration of surface water from streams, ditches, and flood-irrigated fields maintains a shallow water table beneath much of the valley floor where depth to ground water is less than 5 ft (Maurer and Peltz, 1994, sheet 2). Depth to water beneath alluvial fans on the western side of the valley increases to more than 200 ft within 1 mi of the valley floor, whereas depth to water on the eastern side of the valley reaches 200 ft about 3 mi from the valley floor (Maurer and Peltz, 1994, sheet 2).
Ground water flows from the west and east towards the Carson River and then northward (Berger and Medina, 1999). Along the main axis of the valley, water-level gradients range from about 100 ft/mi in the southwestern part of the valley to about 5 ft/mi in the central and northern parts of the valley (calculated from Maurer, 1986, p. 18). Beneath alluvial fans on the western side of the valley, the gradient generally is eastward at about 100 ft/mi, whereas on the eastern side of the valley the gradient generally is westward and ranges from 20 to 100 ft/mi (calculated from Maurer, 1986, p. 18).
Consolidated granitic and metamorphic bedrock surrounding and underlying Carson Valley is relatively impermeable to ground-water flow, although some wells produce sufficient water from fractures for domestic use. In semiconsolidated Tertiary sediments, lenses of sand and gravel are the primary water-bearing features, and probably transmit most ground water that moves through the unit. Unconsolidated sediments that form alluvial fans surrounding the valley floor and underlie the flood plain of the Carson River are the principal aquifers in Carson Valley (Maurer, 1986, p. 17).
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