Estimated water surface elevations for selected monthly flow exceedance percentiles for Firesteel Creek upstream from Lake Mitchell in Davison County, South Dakota, 2021

The City of Mitchell, South Dakota is exploring ways to improve water quality in Firesteel Creek, which flows into Lake Mitchell. One possible way is to construct a wetland on City-owned land in the Firesteel Creek floodplain above Lake Mitchell, allowing sediment deposition and nutrient uptake by aquatic vegetation in the constructed wetland. In order for a constructed wetland to be designed, information on typical flows and water surface elevations of Firesteel Creek throughout the growing season must be considered. This data release documents datasets and procedures used to estimate that information for a study area encompassing the proposed location of the constructed wetland. Monthly duration curves of daily mean flow for April through October were developed from data collected at a streamgage located upstream on Firesteel Creek (06477500, Firesteel Creek near Mouth Vernon, SD). Drainage areas for 06477500 and the study area were determined using South Dakota's StreamStats application. A drainage area ratio adjustment was applied to the 5-, 10-, 20-, 30-, 40-, 50-, 60-, 70-, 80-, 90-, and 95-percent exceedance flows at 06477500 to estimate the corresponding exceedance flows at the study area. Global Navigation Satellite System equipment was used to collect elevation data on the Firesteel Creek channel bottom and at water surface for a set of transects through Firesteel Creek near the proposed wetland location. Elevations for portions of the transects in the floodplain were determined from the one-ninth arc-second National Elevation Dataset. Because the spillway of Lake Mitchell results in backwater conditions within the study area, transects through Lake Mitchell were also determined using the NED and an existing bathymetry map of Lake Mitchell. The data collected at transects were used to develop a hydraulic model using the Hydraulic Engineering Center's River Analysis System (HEC-RAS) software. The hydraulic model was calibrated using water surface elevation data collected during a GNSS survey and the mean streamflow determined for that day. The selected monthly exceedance flows were used as input to the model to estimate water surface elevations for transects in the study area and near the proposed wetland location.