Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Hydrologic Setting

Carson Valley lies in the rain shadow of the Sierra Nevada, with annual precipitation at the town of Minden averaging 8.4 in/yr (period of record 1971–2000, National Oceanic and Atmospheric Administration, 2002, p. 12). In contrast, the top of the Carson Range receives about 40 in/yr and the top of the Pine Nut Mountains receives from 15 to 18 in/yr (Maurer and Halford, 2004, p. 35). From 1987 to 1992 and from 1998 to 2005, conditions were dry with annual precipitation considerably less than average (fig. 6A). The Palmer Drought Severity Index (PDSI, National Oceanic and Atmospheric Administration) is based on long-term weather conditions and provides a cursory indication of regional meteorological wet or dry periods (fig. 6B; National Oceanic and Atmospheric Administration, 2006). The PDSI indicates dry conditions have dominated western Nevada since about 1999.

The hydrology of Carson Valley is dominated by flow of the Carson River. The East and West Forks of the Carson River enter from the southern parts of the valley and flow northward to join near Genoa. The combined flow continues north to leave the valley southeast of Carson City (fig. 2). Flow of the Carson River is diverted across the valley floor through a network of canals and ditches for flood irrigation of crops and native pasture grasses. Thirteen perennial streams drain the Carson Range, whereas only two perennial streams, Buckeye and Pine Nut Creeks, drain the Pine Nut Mountains Valley (Maurer and others, 2004).

Streamflow entering Carson Valley in the East and West Forks of the Carson River, and streamflow leaving the valley in the Carson River near Carson City has been gaged during a common period of record from 1940 to 2005 (stations 10309000, 10310000, and 10311000, respectively). Average annual flow at these three gaging stations was determined for different periods (table 1). Average flow at these stations for water years 1941–70 was very similar to that for the entire common period of record, whereas average flow for water years 1971–2000 was about 3 to 6 percent greater than the common period of record. Average flows for water years 1990–2002 and 1990–2005 range from 0.6 percent less for the West Fork Carson River to almost 6 percent less for the Carson River near Carson City, than for water years 1940–2005.

Infiltration of surface water through streambeds and ditches and beneath flood-irrigated fields maintains a shallow water table beneath much of the valley floor where depth to ground water is less than 5 ft below land surface (fig. 2; Maurer and Peltz, 1994, sheet 2). Depth to water beneath alluvial fans on the western side of the valley quickly increases to greater 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 (fig. 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, ground-water gradients range from about 100 ft/mi in the southwestern part of the valley to about 5 ft/mi in the northern part of the valley. Beneath alluvial fans on the western side of the valley, the ground-water gradient is eastward at about 100 ft/mi, whereas on the eastern side of the valley, gradients are westward and range from 20 to 100 ft/mi (Maurer, 1986, p. 18).

The consolidated granitic and metamorphic bedrock surrounding and underlying Carson Valley are relatively impermeable to ground-water flow, although some wells produce sufficient water from fractures for domestic use. In the semi-consolidated Tertiary sediments, lenses of sand and gravel are the primary water-bearing units, and probably transmit most ground water through the unit. Unconsolidated sediments that form alluvial fans surrounding the valley and that underlie the flood plain of the Carson River are the principal aquifers in Carson Valley (Maurer, 1986, p. 17).

Confined conditions and artesian flow are found in aquifers beneath the valley floor at depths of 200 to 300 ft below land surface (Maurer, 1986, p. 17). Inspection of drillers’ logs has shown that a valley-wide confining unit is not present (Dillingham, 1980, p. 40). Confined conditions are likely the result of discontinuous clay beds 30 to 40 ft thick present at depths of 200 to 300 ft in scattered locations beneath the valley floor. Confined conditions and artesian flow is encountered at shallower depths, less than 100 ft, on the westernmost side of the valley floor. Here, confined heads may result where wells penetrate the toes of coarse-grained alluvial fans that have been buried by fine-grained flood-plain deposits of the Carson River (Maurer, 1986, p. 17).

As part of the overall study of Carson Valley water budgets, water levels have been measured in about 70 wells beginning about December 2004 (selected wells in fig. 7). The USGS has measured some of these wells since 1977, and many were measured during the study by Maurer (1986) in the early 1980s with continued measurements at various times through 2004.

Water levels along the western side of Carson Valley show seasonal fluctuations of 5 to 25 ft during the early 1980s when water levels were measured monthly (fig. 8A, wells 2 and 16), and longer term fluctuations in response to wet and dry periods (see fig. 6) of 5 to 30 ft. Similarly, shallow wells on the valley floor show both seasonal and longer term fluctuations of smaller magnitude, ranging from 2 to about 8 ft, although data from dry years of the late 1980s and from wet years of the late 1990s are not available for many of these wells (fig. 8B). From 1980 to 2006, water levels have fluctuated but overall show no long-term change. Water levels may change temporarily in response to seasonal and annual variations in recharge, but over time, if water levels do not show rising or declining trends, the aquifer is said to be in a state of dynamic equilibrium (Theis, 1940, p. 277).

Water levels in deep flowing artesian wells fluctuate from about 5 to almost 20 ft above land surface (negative values in terms of depth below land surface, fig. 8C) in winter months and from 10 to 20 ft below land surface in response to summer pumping, with water levels in 2005 similar to those during the dry years of the early 1990s (fig. 8C). Water levels in deep irrigation wells (fig. 8D) in 2005 also are similar to those of the early 1990s, however, water levels in 2004 are somewhat lower than in the early 1990s. This may be the result of more severe drought conditions prior to 2004 than in the early 1990s (fig. 6B), or a response to increased pumping.

Water levels in wells near the valley floor show little long-term rise or decline from 1977 to 2006, suggesting that this part of Carson Valley is in a state of approximate dynamic equilibrium. However, on the eastern side of Carson Valley in areas of increased population growth supplied largely by individual domestic wells (fig. 8E), water levels show long-term water-level declines. Water levels in the Johnson Lane area (well 5) have declined about 5 ft lower than during the early 1990s, whereas water levels in Fish Spring Flat (well 10) and Ruhenstroth subdivisions (well 25) have declined about 10 ft. These areas are relatively distant from land irrigated with surface water and recharge is limited to ground-water inflow from the Pine Nut Mountains. Wet conditions during water year 2006 may cause water levels in these areas to rise as increased recharge from the Pine Nut Mountains moves westward.

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