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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. Streamflow in the Walker River is diverted for irrigation, which has contributed to a decline in lake-surface altitude of about 150 ft and an increase in dissolved solids from 2,500 to 16,000 milligrams per liter in Walker Lake since 1882. 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. Accurately determining the bathymetry is one component of a study to improve the water budget for Walker Lake. This report describes the bathymetry of Walker Lake, a comparison of relations between lake-surface altitude, surface area, and storage volume from this study and a study by Rush in 1970, and an estimate of the 1882 lake-surface altitude.

Depth and position data of the lake bottom were collected using an automated system consisting of a single-beam echosounder coupled to a real-time, differentially-corrected GPS. Depth was measured along about 250 mi of transects from February through April 2005. Lidar data and high resolution aerial imagery were acquired around Walker Lake in May and June, 2005 and used to make DEMs. In September 2005 and August 2006, side-scan sonar and divers were used to verify the presence of mounds in the deepest part of the lake that were detected by the single-beam echosounder.

The bathymetric data and DEMs were merged together to create a single map showing land-surface altitude contours delineating areas that are currently or that were submerged by Walker Lake. Paleohydrologic research suggests the lake-surface altitude of Walker Lake has not exceeded 4,120 ft throughout the Holocene. Contours were mapped up to 4,120 ft because of the uncertainty in the historic lake-surface altitude and to aid future paleoclimatic studies. Surface area and storage volume were calculated at 0.5-ft intervals for lake-surface altitudes of 3,851.5–4,120 ft with 3-D surface-analysis software.

Walker Lake is oval shaped with a north-south trending long axis. On June 28, 2005, the lake-surface altitude was 3,935.6 ft, maximum width was 5.5 mi, maximum length was 12.7 mi, maximum depth was 86.3 ft, storage was 1,779,000 acre-ft, and the surface area was 32,190 acres. The minimum altitude from discrete point depths is 3,849.3 ft near the center of Walker Lake.

The lake bottom is remarkably smooth, except for 93 mounds and anomalies ≥3 ft tall with a maximum of height of 32.3 ft. Mounds near the shore and river mouth could be boulders, tree stumps, logs, and other submerged objects. Anomalies occur in the deepest parts of Walker Lake, miles from the shore and river mouth so it is unlikely that these anomalies are boulders or debris from the Walker River. Rocks and concretions that appear to be tufa were observed around the anomalies with a submersible camera. However, side-scan sonar and divers did not verify the presence of the anomalies. Anomalies occur in two northwest trending groups in northern and southern Walker Lake. It is hypothesized that some anomalies indicate spring discharge along faults based on the tufa-like rocks, the northwest trend parallel to and in proximity of mapped faults, and a measured evaporation rate that is about 50 percent higher than the previous estimate. Additional studies are needed to determine what the anomalies are and whether they are related to the hydrology of Walker Lake.

Differences in surface area between this study and the study by Rush in 1970 ranged from -45.6 to 100 percent with a median of 1.0 percent and a mean of 0.1 percent. Differences in storage volume ranged from -150 to 100 percent with a median of 0.5 percent and a mean of 0.0 percent. In general, the bathymetry by Rush in 1970 and from this study are nearly identical throughout most of the range in lake-surface altitude.

The lake-surface altitude in 1882 was estimated to be between 4,080 and 4,086 ft with a probable altitude of 4,082 ft. This estimate compares well with previous estimates of 4,083 and 4,086 ft. Researchers believe the historic highstand of Walker Lake occurred in 1868 and estimated the highstand was between 4,089 and 4,108 ft. By 1882, Mason Valley was predominantly agricultural. The 7–26 ft decline in lake-surface altitude between 1868 and 1882 could partially be due to irrigation diversions during this time.

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