Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Geographic Setting

Carson Valley primarily is in Douglas County, Nevada, about 4 mi south of Carson City, Nevada’s capital (fig. 1). The southern end of the valley extends about 3 mi into Alpine County, California (fig. 1). The floor of the valley is oval-shaped, about 20 mi long and 8 mi wide, and slopes from about 5,000 ft above sea level at the southern end to about 4,600 ft at the northern end. The Carson Range on the western side of the Sierra Nevada rises abruptly from the valley floor with mountain peaks ranging from 9,000 to 11,000 ft, whereas, the Pine Nut Mountains on the eastern side rise more gradually to peaks ranging from 8,000 to 9,000 ft.

The major towns in the valley are Minden and Gardnerville with populations of 2,800 and 3,400, respectively (fig. 2; U.S. Census Bureau, 2003). The subdivisions Gardnerville Ranchos south of Gardnerville and Johnson Lane and Indian Hills north of Minden are growing rapidly, with populations of 11,000, 4,800, and 4,400, respectively (U.S. Census Bureau, 2003). In addition, development is increasing along the eastern and western sides of the valley, and on the valley floor on land that historically has been agricultural. Douglas County as a whole has grown from a population of about 28,000 in 1990 to 41,000 in 2000, an increase of 49 percent (Economic Research Service, 2003).

For purposes of this study, the boundary of the Carson Valley study area was delineated as a subarea of the entire Carson Valley Hydrographic Area (figs. 2 and 3). The study area was selected to include only those parts of the hydrographic area connected by permeable aquifer materials capable of transmitting ground water to aquifers beneath the floor of Carson Valley. Along the southern boundary, the headwaters of the West and East Forks of the Carson River were not included in the study area because bedrock underlies the points where the West and East Forks of the Carson River cross the study area boundary, restricting ground-water inflow (fig. 3). The study area boundary (figs. 2 and 3) covers 253,570 acres, or about 396 mi2.

The valley floor is covered with native pasture grasses, crop lands of primarily alfalfa, and near the northern end of the valley, phreatophytes such as greasewood, rabbitbrush, and big sage. The distribution of these types of vegetation and other types of land use on the floor of Carson Valley were delineated for this study on a land-use map (fig. 4). The land-use map was developed using imagery collected in July 2004 by the Carson Valley Conservation District (BAE SYSTEMS Advanced Technologies, Inc., 2004). The imagery collected had a nominal ground sampling area of about 3 ft2 and was digitized on screen to determine the distribution and areas of vegetation and land-use types. The initial map was field checked during the summer of 2005 and updated to include changes from July 2004 when the imagery was flown. The areas of selected vegetation and land-use types, and of selected geologic units listed in this report were calculated using Geographic Information System (GIS) software. Areas determined from the imagery were considered approximations only, because it was not feasible to field check all digitized polygons. In addition, the map and areas should be considered a snapshot in time because of the rapidly changing land use in Carson Valley.

At altitudes above the valley floor on the western side of the valley, bitterbrush and sagebrush cover steep alluvial fans, and manzanita and ponderosa pine cover the slopes of the Carson Range. Alluvial fans and foothills of the Pine Nut Mountains on the eastern side of the valley are covered with sage and rabbitbrush, and pinyon and juniper are found on the Pine Nut Mountains.

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