Dean, Graduate Education and Sponsored Programs

ii This report is part of a watershed study for the Spring Creek basin with the primary focus on Sheridan Lake. It describes the findings of a one-and-a-half year study to determine the current trophic state index of Sheridan Lake and ways to reduce it if necessary. Gages throughout the basin were used to monitor flow. This information was combined with monthly water quality samples and storm water quality samples to determine mass loading of nutrients along the length of Spring Creek. In-lake sampling at two sites identified phosphorus as the limiting nutrient. It also showed that the lake becomes depleted of oxygen below 25 feet by mid July. The drop in oxygen was accompanied by a rise in nutrient levels in the hypolimnion under anoxic conditions. Statistical analysis of water quality data indicates that Hill City and Palmer Gulch are major sources of phosphorus to Spring Creek and eventually to Sheridan Lake. Both showed large increases in phosphorus and other nutrient concentrations during storm events. Modeling of the lake determined that a 43 % reduction of incoming phosphorus concentration would be necessary to reach the target Trophic State Index (TSI) of 45. In order to reach this level, estimates were made of potential reduction of phosphorus loading in the basin based on implementation of best management practices such as storm water management and stream bank stabilization