Evaluation of Groundwater Flow in the Middle Carson River Basin, Eagle, Dayton, and Churchill Valleys, Nevada

The USGS Nevada Water Science Center (NVWSC) has developed a numerical groundwater-flow model calibrated by a variety of historical observations related to the groundwater and surface-water systems along the Middle Carson River Basin (MCRB).  The model is designed, through appropriate selection of packages available with MODFLOW-NWT, to help water managers assess the possible future impacts of alternative management decisions made today in response to a growing need for water, driven in large part by residential and light industrial development.

The USGS NVWSC 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.  In 2008, the USGS NVWSC and the Bureau of Reclamation began a cooperative study to understand the complex interaction between groundwater and surface water in the MCRB (the river reach extending from the USGS streamflow gaging station on the south side of Carson City to just downstream of Lahontan Reservoir Dam). 

The first phase of the study compiled information describing

• the hydrogeologic framework in which the Middle Carson River (MCR) resides,
• how precipitation is partitioned into runoff and infiltration in twelve perennial and ephemeral tributaries along the MCR,
• key characteristics of a complex surface water network and its historical management, and
• supported the ongoing collection of surface-water flow and groundwater hydraulic head observations that guided model calibration. 

The second phase of the study focused on

• developing a regional groundwater model for the Middle Carson River Basin (MCR model) that synthesizes the broad body of knowledge learned about the MCRB from data collected as part of this and previous studies, and 
• evaluating potential impacts of present and future groundwater withdrawals in Eagle, Dayton and Churchill Valleys on streamflow of the Carson River.

A complete description of the MCR model including calibration and alternative management scenarios will be available in an upcoming USGS Scientific Investigations Report.

DATA

The MCR model was built based on data collected by the USGS NVWSC 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 NVWSC 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.  Additional data needs were addressed in a cooperative study with Lyon County.

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, 
• lithologic data from drillers’ logs,
• groundwater levels and temperature adjacent to the Carson River and Lahontan Reservoir, and
• streamflow and in-stream temperature to determine the spatial and temporal distribution of recharge from ephemeral streams tributary to valley floors.  

These data have been and will continue to be used for calibration of the MCR model. Collectively, this data helped determine the spatial and temporal distribution of gaining and losing reaches along the river and surrounding the reservoir.