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Scientific Investigations Report 2007–5012

Scientific Investigations Report 2007–5012

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Walker Lake lies within a topographically closed basin in west-central Nevada and is the terminus of the Walker River (fig. 1). Walker River is the main source of inflow, but small tributaries from adjacent mountains and subsurface flow also discharge to Walker Lake (Thomas, 1995). The only outflow from Walker Lake occurs by evaporation from the lake surface. Since the latter half of the 1800s, much of the streamflow in the Walker River has been diverted for irrigation (Russell, 1885). Since 1882 diversions have reduced flow into Walker Lake which contributed to a decline in lake-surface altitude of about 150 ft and an increase in dissolved solids from 2,500 to 16,000 mg/L since 1882 (fig. 2). The increase in salinity threatens the fresh-water ecosystem and survival of the Lahontan cutthroat trout, a species listed as threatened under the Endangered Species Act.

Section 2507 of Public Law 107-171 (2002 Farm Bill) provided $200,000,000 to be used by the Secretary of Interior, acting through the Commissioner of Reclamation, to provide water to at-risk natural desert terminal lakes. This bill was later amended under Public Law 108-7, section 207 to include language “Restoration of fish, wildlife, and associated habitats in watersheds of certain lakes”. The amendment specified that only Pyramid, Summit, and Walker Lakes in Nevada were to be considered under Section 2507, Public Law 107-171.

In response to the 2002 Farm Bill, the U.S. Geological Survey (USGS), in cooperation with the Bureau of Reclamation, began a study to better estimate the water budget for Walker Lake and to develop the capability to predict how changes in irrigation practices will affect flows in the lower Walker River (Lopes, 2005). Bathymetry is the measurement of water depth, which can be used to make contours of equal water depth and equal altitude of submerged land. Bathymetry is needed to relate the water-surface altitude to the surface area and storage volume of the water body. Accurately determining the surface area and the evaporation rate is necessary to quantify outflow from Walker Lake. Rush (1970) mapped the bathymetry of Walker Lake using 877 soundings made in 1957. Rush (1970) and Thomas (1995) used this bathymetry to estimate the volume of water that evaporates from Walker Lake. To improve on these water budgets, bathymetry was measured using instruments that automatically collect thousands of accurate data points.

Purpose and Scope

This report presents a description of the updated bathymetry of Walker Lake; comparisons of relations between lake-surface altitude, surface area, and storage volume from this study and Rush (1970); and an estimate of the 1882 lake-surface altitude. Relations between lake-surface altitude, surface area, and storage volume were made to an altitude of 4,120 ft because of uncertainty in the earliest (1882) measured depth of Walker Lake (Adams, 2007).

Description of Walker River Basin

The Walker River Basin is about 4,050 mi2 and straddles the California-Nevada border. About 25 percent (1,002 mi2) of the basin is in California. Most precipitation in the basin occurs as snow in the Sierra Nevada Range, which has peaks that reach a maximum altitude of 12,264 ft at Matterhorn Peak. Snowmelt from the Sierra Nevada and other ranges flows down the east and west forks of the Walker River, which merge into the Walker River in Mason Valley, Nev., and continues flowing downstream into the northern end of Walker Lake. Walker Lake is bounded on the west by the Wassuk Range and on the east by the Gillis Range. The Wassuk Range rises abruptly from the lake by more than 7,000 ft and has a maximum altitude of 11,239 ft at Mt. Grant. The Gillis Range is not as steep and has a maximum altitude of 7,887 ft. The lowest altitude is 3,849.3 ft at the deepest part of Walker Lake.

Walker Lake is a depression within the Walker Lane, a broad zone of mostly north-northwest trending faults that extends from near Las Vegas, Nev. to Honey Lake Valley, Calif. (Bonham, 1969; Stewart, 1988). The Walker Lane is a complex fault system consisting of both extensional and right-lateral, strike-slip faulting and as much as 40 mi of lateral displacement in the Walker Lake area (Wesnonsky, 2005). Several faults have been mapped adjacent to and north of Walker Lake. Some of these faults or associated fault strands could underlie Walker Lake.

Mason Valley has long had the most agricultural activity in the Walker River Basin and is one of the most productive agricultural areas in the State. Most of Mason Valley was mapped as agricultural by Russell (1885), who described “irrigable lands where abundant harvests are annually secured.” Russell (1885) did not map agriculture in Smith and Antelope Valleys, which have had irrigation diversions at least since 1931 and 1943, respectively (Pahl, 2000). On June 4, 1881, Russell (1885) measured 400 ft3/s near Schurz, Nev., about 3 miles upstream from the mouth of the Walker River. In October 1882, the river at this location was dry and “little, if any, water reached the lake from this source.” According to Russell (1885), the lack of streamflow was “due in a great measure to the extensive use of its waters for irrigation in Mason Valley.”

A total of 88,600 acres of irrigated land was mapped in the Walker River Basin using Landsat satellite imagery acquired in 2000. Total irrigated land included 39,100 acres (44 percent) in Mason Valley; 18,900 acres (21 percent) in Smith Valley; 15,900 acres (18 percent) upstream from Bridgeport Reservoir; 7,700 acres (9 percent) in Antelope Valley; 3,500 acres (4 percent) along the east fork of the Walker River below Bridgeport Reservoir; 3,000 acres (3 percent) in the Walker River Indian Reservation; and 500 acres (<1 percent) south of Hawthorne, Nev. The primary crop is alfalfa, except for upstream from Bridgeport where fields are irrigated for pasture grass. Other crops that are grown in the basin include onion, garlic, corn, and winter wheat.

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