Hydrologic Information for Evaluation of Reservoir Management Strategies for Sheridan Lake to Enhance Fish Habitat in Spring Creek

Executive Summary

Sheridan Lake is located in the central Black Hills of western South Dakota and is formed by a dam on Spring Creek. Historically the dam has been operated primarily as a simple pass-through system, in which unregulated discharge passed over the spillway. The dam recently was retrofitted with a control valve system that allows controlled releases to be made from a fixed depth of about 60 feet.

The fishery resource in Sheridan Lake and in Spring Creek downstream from the lake is managed by the South Dakota Department of Game, Fish and Parks (GF&P). GF&P is proposing to collaborate with USFS to manage future releases from Sheridan Lake for purposes of enhancing fish habitat and other ecosystem values in downstream reaches of Spring Creek. Spring Creek is generally considered perennial upstream from Sheridan Lake; however, low-flow conditions can approach or reach zero flow. Evaporation from Sheridan Lake can approach or exceed inflows during low-flow periods; thus, zero flow or conditions approaching zero flow can occur relatively frequently in downstream reaches.  Thermal heating of the water during low-flow conditions is especially harmful to trout populations and the associated fishery in reaches of Spring Creek downstream from Sheridan Lake; thus, augmentation of downstream flow rates with relatively small quantities of cool water released at depth has excellent potential to sustain and enhance fish habitat. Evaluating the feasibility of proposed operational alternatives and development of specific operating criteria is prerequisite to future implementation activities. GF&P has requested assistance from the U.S. Geological Survey (USGS) in compilation and analyses of relevant hydrologic information to be used for these purposes.

Objectives

The purpose of the project is to compile and analyze hydrologic information relevant to evaluation of reservoir management strategies for Sheridan Lake. Resulting information would be used by GF&P and USFS for evaluating the feasibility of proposed operational alternatives and for development of specific operating criteria. The scope of the project recently was modified to include collection of limited streamflow, lake-level, and precipitation data, which will facilitate improvement of a detailed water budget, especially through more precise quantification of evaporation from the lake. Specific objectives of the project are to (1) compile and characterize critical components of a hydrologic budget for Sheridan Lake and downstream reaches of Spring Creek; and (2) develop a hydrologic model for simulation of Sheridan Lake drawdowns under hypothetical future conditions of inflow and outflow. Critical water-budget components include stream inflow, precipitation, and evaporation, and compilation efforts probably will include synthesis of some data sets. Statistical characterization of data sets will provide useful information for evaluation of operational alternatives and development of operating criteria. These data sets will be critical to development of a hydrologic model for simulation of Sheridan Lake drawdowns, which will focus on low-flow conditions. Simulation output will be used for development of probability thresholds for key hydrologic events associated with reservoir management strategies.

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