TOTAL MAXIMUM DAILY LOAD DEVELOPMENT FOR THE LOWER CHEYENNE RIVER WATERSHED

The Cheyenne River in western South Dakota is currently listed as impaired for total suspended solids (TSS) and fecal coliform bacteria. A Total Maximum Daily Load (TMDL) assessment project for the Cheyenne River Watershed in western South Dakota was initiated in the spring of 2007. The focus area of this project was the Cheyenne River Watershed below Angostura Reservoir, near Hot Springs, South Dakota, to the mouth of the river where it meets Oahe Reservoir.

A major emphasis of the TMDL project was to identify contributing areas of sediment loadings to the Cheyenne River and to evaluate natural background, while linking sediment loading to stream health. This was accomplished through four main tasks: design and implementation of a detailed water-quality monitoring plan; development of a watershed model using the program Hydrologic Simulation Program Fortran (HSPF), collection and analysis of benthic macroinvertbrate biological data, and collection and analysis of stream physical habitat data.

The Cheyenne River has a large natural loading of sediment, largely originating from the badlands region of western South Dakota. The Cheyenne River has four distinct reaches. The first reach, from Angostura Dam to Cedar Creek near Redshirt Table, has relatively low loading for both TSS and fecal coliforms. The second reach, from Cedar Creek to near Wasta, is a transition reach in water quality for both TSS and fecal coliforms, where loading increases dramatically. The third reach, from Wasta to the confluence of the Belle Fourche River, has a shift in stream morphology and sediment transport processes. The final reach, from the Belle Fourche River to the mouth, has consistent increases in both TSS and fecal coliform loading. Biologic condition, based on Index of Biotic Integrity (IBI) scores calculated from benthic macroinvertebrate data, increases upstream to downstream, with the highest IBI scores occurring in the lowest reaches of the Cheyenne River.