Abstract

Changes in land use and water use and increasing development of water resources in the middle Carson River basin may affect flow of the river and, in turn, affect downstream water users dependent on sustained river flows to Lahontan Reservoir. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, began a study in 2008 of the middle Carson River basin, extending from Eagle Valley to Churchill Valley. Various types of geologic and hydrologic data were compiled from previous studies, collected for this study, and compiled and analyzed to provide a framework for development of a numerical model of the groundwater and surface-water flow systems of the basin.

Geologic units that are assumed to have similar hydrologic characteristics were grouped into hydrogeologic units comprised of consolidated rocks of pre-Cenozoic age that underlie a unit of consolidated volcanic rock and semi-consolidated sediments of Tertiary age. The principal aquifer in the study area is comprised of unconsolidated sediments of Quaternary age. The Quaternary sediments include alluvial fan, fluvial, and lake sediments, and were grouped into a basin-fill hydrogeologic unit that overlies the pre-Cenozoic and Tertiary hydrologic units.

The thickness of the combined section of Tertiary volcanic and sedimentary rocks and Quaternary basin-fill deposits previously was estimated to range from zero where pre-Cenozoic rocks are exposed to greater than 10,000 feet in the Bull Canyon subbasin, and greater than 6,000 feet on the western side of Churchill Butte and beneath the Desert Mountains. The thickness of Quaternary basin-fill sediments was estimated using gravity data and lithologic descriptions from driller’s logs. The most permeable parts of basin-fill sediments are greater than 1,000 feet thick in the Carson Plains subbasin, greater than 800 feet and 600 feet thick in the western and northeastern parts of the Stagecoach subbasin, and greater than 1,000 feet and 800 feet thick in the northern and southern parts of Churchill Valley, respectively.

The distribution of aquifer properties was estimated for basin-fill sediments using slug-test and aquifer test data, and the lithologic descriptions of previously mapped geologic units. Slug-test data show hydraulic conductivity is greater than 10 to greater than 100 feet per day for fluvial sediments near the flood plain, less than 10 feet per day for basin-fill sediments outside the flood plain, and less than 1 foot per day for consolidated rocks. Estimates of transmissivity exceed 20,000 feet squared per day near the Carson River in Dayton, Churchill, and western Lahontan Valleys and in the northern part of the Stagecoach subbasin, and exceed 10,000 feet squared per day in the western part of Churchill Valley. A transmissivity of 90,000 feet squared per day was estimated from results of an aquifer test in the Carson Plains subbasin, indicating that permeable gravel and cobble zones at depths greater than 400 feet supplied water to the pumping well. Estimates of specific yield ranged from less than 1 to 2 percent for most consolidated rocks, from 1 to 15 percent for semi-consolidated Tertiary sediments, and from 10 to 40 percent for unconsolidated basin-fill sediments.

Water-level altitude maps based on measurements at about 300 wells in 2009 show water levels have declined as much as 70 feet since 1964 on the northwestern side of Eagle Valley, about 10 feet since 1995 near Dayton in the Carson Plains subbasin, and from 5 to 10 feet since 1982 in the western and northeastern parts of the Stagecoach subbasin and the northwestern part of Churchill Valley. The declines are likely the result of municipal and agricultural pumping. The maps show a groundwater divide between the Carson Plains and Stagecoach subbasins, and a continuous hydraulic gradient between the Stagecoach subbasin and Churchill Valley. Groundwater flow directions are uncertain beneath parts of the boundary of Churchill Valley. The altitude of the top of pre-Cenozoic rocks shows thick sections of saturated Tertiary rocks and sediments south of the Dead Camel Mountains and beneath the eastern part of the Desert Mountains through which groundwater flow between Churchill Valley, Mason Valley, and Lahontan Valley may take place. North of Lahontan reservoir, beneath the Dead Camel Mountains, and beneath the southern part of Adrian Valley, the altitude of pre-Cenozoic rocks indicates groundwater flow between the three valleys is minimal.

Streamflow measurements, supported by data on the deuterium content and specific conductance of surface-water samples, indicate a loss of Carson River streamflow in the Riverview subbasin, streamflow gains in the Moundhouse subbasin and the eastern part of the Carson Plains subbasin, and streamflow losses in the Bull Canyon subbasin. Comparisons of fluctuations in groundwater levels to those in stream stage in the Carson Plains subbasin indicate that streamflow lost to infiltration from the Carson River, from irrigation ditches, and from irrigated fields is an important source of groundwater recharge. Fluctuations in groundwater levels compared with the stage of Lahontan Reservoir in Churchill Valley indicate losses to infiltration from the reservoir during high stage and groundwater seepage to the reservoir during low stage.

First posted May 2, 2011