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Scientific Investigations Report 2007–5012

Scientific Investigations Report 2007–5012

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Bathymetry of Walker Lake

Description of Walker Lake

Walker Lake is oval shaped with a north-south trending long axis (pl. 1). The minimum altitude from discrete point depths is 3,849.3 ft near the center of Walker Lake. At a lake-surface altitude of 4,120 ft, the storage volume is 11,837,000 acre-ft and the surface area is 81,360 acres (appendix A). 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 lake bottom is steepest along the western shore, moderately steep along the eastern shore, shallowest at the northern and southern ends, and becomes fairly flat about 0.3 mi from the western and eastern shores (pl. 1, fig. 6A-D). Scour channels up to 425 ft wide were observed along the most northern transect near the mouth of the Walker River (fig. 6A). Deltaic sediments created a broad mound up to 6.5 ft tall and 1 mi wide that extends about 2.8 mi south from the mouth of the Walker River. Algal growth abruptly appeared near the river mouth and in water about 20-ft deep along much of the eastern shore. Less algal growth was observed in other parts of the lake. The lake bottom along the transect at universal transverse mercator (UTM) northing 4275000 has a sudden drop of about 4 ft, which could be fault offset (fig. 6C). The rest of the lake bottom is remarkably smooth, except for mounds near the shore and river mouth.

Mounds are indicated by sonar return signals that abruptly ramp up and down with no return signals beneath the mounds. Logs, fish, or other objects floating near the bottom of the lake are distinguished from mounds because return signals appear underneath the objects. Mounds near the mouth of the Walker River could be stumps of trees that grew during long periods of low lake-surface altitude (Adams, 2007) or partially buried logs carried in from the river. Some of the mounds, especially those near the western shore, likely are boulders that tumbled into the lake or were deposited by prograding alluvial fans during long periods of low lake-surface altitude. Using a submersible camera, piles of boulders south of the town of Walker Lake were observed along the shore and in water about 30-ft deep. Large piles of boulders also could be at the western end of the transect at UTM northing 4276000 (fig. 6C) and the southern end of the transect at UTM easting 348000 (fig. 6D).

The echosounder detected what appeared to be mounds in the deepest parts of Walker Lake, miles from the shore and river mouth (figs. 6A-D). Sonar return signals from the anomalies were the same as signals from boulders or other solid objects lying on the bottom (fig. 7). However, side-scan sonar and divers did not verify their presence. The vertical exaggeration in figure 7 makes anomalies appear similar to tufa towers along the shores of Mono Lake (fig. 8). However, about 75 percent of the mounds and anomalies in Walker Lake are low, broad features with a width that is at least twice the height. Tufa typically forms as tall structures along the margins of lakes and not in the deepest waters.

The three anomalies in figure 7 were located on March 3 and again on March 4, 2005, when the lake bottom was viewed with a submersible camera. Rocks were observed that appeared to be tufa and the lake bottom was pock marked with many holes. In August 2006, divers determined that the holes were burrows made by mayfly larvae and red worms. The lake was well mixed when bathymetry was done and there is no indication that the equipment malfunctioned, so it is unknown what caused the anomalous return signals.

A number of objects and features were seen during the side-scan sonar survey. Numerous strong reflections were imaged almost immediately upon starting the survey, but their shadows were separate from their reflection. The pattern of the reflections and separation from their shadows indicated the reflections likely were schools of fish. Images of large boulders were taken along the cliffs north of the town of Walker Lake (fig. 9). The line in the center of figure 9 is the course of the towfish. Side-scan sonar obtains images on both sides of the towfish but not underneath it, which is why there is a dark area in the center of figure 9. Images were taken of a few mounds, but generally these were less than 3-ft tall.

A total of 93 mounds and anomalies ≥3-ft tall with a maximum height of 32.3 ft were located in Walker Lake (fig. 10; appendix B). East-west and north-south transects detected mounds and anomalies in the same areas in the northern and southern parts of the lake. The two groups have a roughly northwest trend and about the same number of mounds. The northern group extends from the northwestern to mid-eastern sides of Walker Lake and is at the southern end of a northwest trending fault that crosses the Wassuk Range. This fault or associated fault strands could continue southeast and underlie the lake. The southern group extends from about 2.5 mi northeast of the town of Walker Lake to the southern end of the lake. The steep slope north of the town of Walker Lake likely is a fault scarp (fig. 6B). The northwest trend parallel to and in proximity of mapped faults suggests that anomalies could be related to faults.

The measured evaporation rate from Walker Lake is almost 6 ft/yr (Allander and others, 2006), which is nearly 50 percent more than 4.1 ft/yr estimated by Harding (1965) and used in previous water budgets (Rush, 1970; Thomas, 1995). To balance the higher evaporation rate, more water must be flowing into the lake from sources other than the Walker River than previously estimated. 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 the measured evaporation rate. Schaefer (1980) estimated 11,000 acre-ft/yr of water discharges from the alluvial aquifer into northern Walker Lake. This discharge could be occurring along a fault, forming the northern group of northwest-trending anomalies. Mt. Grant could be the source of water if the southern anomalies are active springs. The town of Walker Lake was built on an alluvial fan formed by sediments from the Cottonwood Creek drainage, the highest and largest drainage along the eastern Wassuk Range and just west of the southern group (fig. 10). Snowmelt could infiltrate as it flows across the alluvial fan, flowing through layers of sand and gravel that deposited on the lake bed. Offset of these layers by faults could cause ground water to discharge, forming the southern group of anomalies. Additional studies need to be done to determine what the anomalies are and whether they are related to the hydrology of Walker Lake.

Comparison to Bathymetry by Rush

The bathymetry from this study was compared to the bathymetry of Rush (1970) to determine how much error could be in previous water budgets due to inaccurate surface-area estimates (Rush, 1970; Thomas, 1995). Comparisons were made for lake-surface altitudes of 3,851.5–4,083 ft, which is the highest altitude that surface area and storage volume calculated for by Rush (1970; fig. 11). Differences between the two data sets were calculated assuming values estimated from this study were the most accurate value.Differences in surface area 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. The largest differences in surface area and storage volume occur at lake-surface altitudes <3,890 ft. Most differences at higher altitudes are between -1 and 1 percent.

A paired t-test indicated the bathymetry of Rush (1970) was statistically different than the bathymetry from this study and slightly overestimated surface area and storage volume. However, the two bathymetries are nearly identical throughout most of the range in lake-surface altitude (fig. 11). Thus, errors in surface area contributed little to errors in evaporation of previous water-budget estimates. The small difference between the two bathymetries largely is due to the simple oval shape and smooth lake bottom. The small difference also is evidence of the accuracy of the soundings made in 1957 and Rush’s (1970) computation of the bathymetric and topographic data.

Estimation of 1882 Lake-Surface Altitude

The earliest (1882) recorded depth of Walker Lake was 225 ft by Russell (1885). The location of this measurement, estimated from Russell’s map on plate XV, was about latitude 38o 41.7’ and longitude 118 o 42.1’. Unknown to Russell, this was not the deepest part of the lake. Russell did not state what the lake-surface altitude was at the time of the measurement. The altitude of the lake bottom within 0.62 mi of the location ranges from 3,854.5 ft to 3,860.8 ft with an average of 3,857 ft and standard deviation of 1.3 ft. Assuming Russell’s sounding was accurate, the lake-surface altitude in 1882 was between 4,080 ft and 4,086 ft with a probable altitude of 4,082 ft. This estimate of the 1882 lake-surface altitude compares well to the estimate of 4,083 ft by Rush (1970). Harding (1965) estimated the 1882 lake-surface altitude was 4,086 ft, which is within the range of possible altitudes.

Researchers estimate that Walker Lake reached its historic highstand in 1868. Previous studies estimated the 1868 lake-surface altitude was 4,089 ft (Harding, 1965), 4,101 ft (Davis, 1982; Link and others, 1985), and 4,108 ft (Blair and McPherson, 1994). Adams (2007) notes that, because the 1860s was a wet decade, Walker Lake could have reached a highstand of 4,108 ft in 1868 and declined to 4,101 ft by 1882. However, an 1882 lake-surface altitude of 4,101 ft is well outside the possible range. Assuming lake-surface altitude in 1882 was 4,082 ft, Walker Lake declined about 7 to 26 ft between 1868 and 1882. By the time Russell (1885) reached the area, Mason Valley was predominantly agricultural. The difference in lake-surface altitude between 1868 and 1882 could at least partly be due to irrigation diversions during this time.

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