ABSTRACT

The Walker River is the main source of inflow to Walker Lake, a closed-basin lake in west-central Nevada. Between 1882 and 2008, agricultural diversions resulted in a lake-level decline of more than 150 feet and storage loss of 7,400,000 acre-ft. Evaporative concentration increased dissolved solids from 2,500 to 17,000 milligrams per liter. The increase in salinity threatens the survival of the Lahontan cutthroat trout, a native species listed as threatened under the Endangered Species Act. This report describes the hydrologic setting of the Walker River basin and a conceptual hydrologic model of the relations among streams, groundwater, and Walker Lake with emphasis on the lower Walker River basin from Wabuska to Hawthorne, Nevada.

The Walker River basin is about 3,950 square miles and straddles the California–Nevada border. Most streamflow originates as snowmelt in the Sierra Nevada. Spring runoff from the Sierra Nevada typically reaches its peak during late May to early June with as much as 2,800 cubic feet per second in the Walker River near Wabuska. Typically, 3 to 4 consecutive years of below average streamflow are followed by 1 or 2 years of average or above average streamflow.

Mountain ranges are comprised of consolidated rocks with low hydraulic conductivities, but consolidated rocks transmit water where fractured. Unconsolidated sediments include fluvial deposits along the active channel of the Walker River, valley floors, alluvial slopes, and a playa. Sand and gravel deposited by the Walker River likely are discontinuous strata throughout the valley floor. Thick clay strata likely were deposited in Pleistocene Lake Lahontan and are horizontally continuous, except where strata have been eroded by the Walker River. At Walker Lake, sediments mostly are clay interbedded with alluvial slope, fluvial, and deltaic deposits along the lake margins. Coarse sediments form a multilayered, confined-aquifer system that could extend several miles from the shoreline.

Depth to bedrock in the lower Walker River basin ranges from about 900 to 2,000 feet. The average hydraulic conductivity of the alluvial aquifer in the lower Walker River basin is 10–30 feet per day, except where comprised of fluvial sediments. Fluvial sediments along the Walker River have an average hydraulic conductivity of 70 feet per day. Subsurface flow was estimated to be 2,700 acre-feet per year through Double Spring. Subsurface discharge to Walker Lake was estimated to be 4,400 acre-feet per year from the south and 10,400 acre-feet per year from the north.

Groundwater levels and groundwater storage have declined steadily in most of Smith and Mason Valleys since 1960. Groundwater levels around Schurz, Nevada, have changed little during the past 50 years. In the Whisky Flat area south of Hawthorne, Nevada, agricultural and municipal pumpage has lowered groundwater levels since 1956. The water-level decline in Walker Lake since 1882 has caused the surrounding alluvial aquifer to drain and groundwater levels to decline.

The Wabuska streamflow-gaging station in northern Mason Valley demarcates the upper and lower Walker River basin. The hydrology of the lower Walker River basin is considerably different than the upper basin. The upper basin consists of valleys separated by consolidated-rock mountains. The alluvial aquifer in each valley thins or pinches out at the downstream end, forcing most groundwater to discharge along the river near where the river is gaged. The lower Walker River basin is one surface-water/groundwater system of losing and gaining reaches from Wabuska to Walker Lake, which makes determining stream losses and the direction and amount of subsurface flow difficult.

Isotopic data indicate surface water and groundwater in the lower Walker River basin are from two sources of precipitation that have evaporated. The Walker River, groundwater along the Wassuk Range, and Walker Lake plot along one evaporation line. Groundwater along the Gillis Range and Calico Hills plots along a different evaporation line that indicates more intense evaporation prior to recharge and that these are not significant sources of water to Walker Lake.

Groundwater in alluvial aquifers generally flows downvalley with flow towards the river in gaining reaches and away from the river in losing reaches. The Walker River is mostly gaining in Smith Valley and losing in Mason Valley. In the lower Walker River basin, the river is losing for most of the reach between Wabuska and Cow Camp upstream from Weber Reservoir and gaining from Cow Camp to Little Dam about 2 miles downstream from Weber Reservoir. Even though most of the reach between Wabuska and Cow Camp is losing, infiltration seems to be small. Some infiltrated water flows north into Churchill Valley through the Wabuska lineament, a zone of northeast-trending faults.

The Walker River is losing for most of the reach between Little Dam and the Lateral 2-A streamflow-gaging stations. Stream infiltration and induced recharge in irrigated fields have created a groundwater divide that extends southeast from Schurz, Nevada. Groundwater south of the divide flows towards Walker Lake. Groundwater east of the divide flows towards Double Spring and out of the Walker River basin.

Most of the reach from Lateral 2-A to Walker Lake is gaining although streamflow can infiltrate along this reach. A maximum loss of 8,000 acre-feet per year was measured during the 2005 spring runoff which followed a 5-year drought. No additional losses were measured during 2006 even though there was continuous flow, indicating bank and aquifer storage had been filled.

Water-table contours and upward vertical gradients indicate Walker Lake is the final discharge point for groundwater in the lower Walker River basin.