Evaluation of Groundwater Flow, Middle Carson River Basin
USGS is collecting groundwater levels and high-altitude precipitation data in the Dayton and Churchill Valley hydrographic areas in cooperation with the Carson Water Subconservancy District. This work began in 1997 to provide a data set for use in the development of a groundwater flow model for the Dayton Valley hydrographic area. The data show long-term water-level declines caused by groundwater withdrawals in the eastern part of the Dayton Valley hydrographic area, but relatively little change elsewhere. Previous work in the Dayton Valley hydrographic area shows groundwater flow directions near the boundaries of the valley are uncertain. A better understanding of the hydrogeologic controls on groundwater flow is needed before a realistic conceptual model of the flow system can be developed. The water-level data also show that groundwater levels as distant as 8,000 feet from Lahontan Reservoir in the Churchill Valley hydrographic area fluctuate in response to changes in reservoir stage. This suggests that there is considerable interaction between groundwater and Lahontan Reservoir. Further studies are required to gain a better understanding of these interactions.
The U.S. Geological Survey (USGS) has modeled, with either PRMS or MODFLOW, several areas within the Carson River Watershed, including part of the upper catchment, the unconsolidated basin fill sediments of Carson Valley, and perennial and ephemeral sub-basins along the Carson River in Carson, Eagle, Dayton, and Churchill Valleys.
Recent USGS modeling efforts have focused on the Middle Carson River, or the river reach extending from the USGS flow monitoring gage on the south side of Carson City to just downstream of Lahontan Reservoir Dam. Currently, this model includes groundwater and surface water components as simulated by the USGS’s groundwater modeling software MODFLOW. These models are built based on data collected by the USGS in cooperation with numerous local entities. Groundwater levels and high-altitude precipitation in the Dayton and Churchill hydrographic areas were collected in cooperation with the Carson Water Subconservancy District. A data compilation phase completed by the USGS in cooperation with the U.S. Fish and Wildlife Service, Churchill County, and the Truckee-Carson Irrigation District served as a foundational piece-of-the-puzzle in developing the groundwater models for Carson, Eagle, Dayton, and Churchill Valleys. Data compiled under this project included historic groundwater-level fluctuations, groundwater withdrawals and distribution, streamflow diversions, main stem flow of the Carson River at existing gages, and lithologic data from drillers’ logs. These data have been and will continue to be used for calibration of the regional ground-water flow model currently being developed for the Middle Carson River.
Data needs not addressed by these studies were subsequently addressed in a follow-up study completed in cooperation with Lyon County, wherein additional data was gathered to refine the understanding of surface-water/groundwater interactions for the middle Carson River Basin. These data collection efforts included additional observations of ground-water levels and temperature adjacent to the Carson River and Lahontan Reservoir and the collection of streamflow and in-stream temperature to determine the spatial and temporal distribution of recharge from ephemeral streams tributary to valley floors. Collectively, this data helped determine the spatial and temporal distribution of gaining and losing reaches along the river and surrounding the reservoir.
Demand for water resources in the Carson River basin is increasing due to steady population growth and the resulting development and changes in land and water use throughout the middle part of the basin. Agricultural land is being urbanized while land not previously irrigated may be converted for agricultural use. The effects of land- and water-use changes on surface-water and ground-water resources are uncertain. The middle Carson River Basin includes the hydrographic areas of Carson Valley, Eagle Valley, Dayton Valley, and Churchill Valley upstream from Lahontan Dam. Rapid growth, particularly in Carson Valley and Dayton Valley, has caused increases in ground-water withdrawals, and the current distribution of municipal supply wells scattered throughout the valleys may change to localized well fields. Additionally, surface and ground water currently used for agriculture will likely be increasingly used for municipal supply, which will change the rates and locations of groundwater recharge and discharge.
Jeton, A.E., and Maurer, D.K., 2011, Precipitation and runoff simulations of select perennial and ephemeral watersheds in the middle Carson River basin, Eagle, Dayton, and Churchill Valleys, west-central Nevada: U.S. Geological Survey Scientific Investigations Report 2011-5066, 44 p.
Maurer, D.K., 2011, Geologic framework and hydrogeology of the middle Carson River basin, Eagle, Dayton, and Churchill Valleys, west-central Nevada: U.S. Geological Survey Scientific Investigations Report 2011-5055, 62 p.