Analysis of Vermillion River Stream Flow Data (Dakota and Scott Counties, Minnesota)

As part of an effort to characterize the response of the Vermillion River to surface runoff, the flow records from seven gaging stations were analyzed to determine the data quality, typical low flows, and the contribution of major tributaries to the total flow. The flow record from the U.S. Geological Survey gaging station at Empire appears to give the most reliable flow data and has the longest record (33 years). An effort by the Minnesota Department of Natural Resources (MNDNR) to recalibrate other Vermillion River flow gaging stations appears to have resulted in self-consistent data for 2007 flows in the main stem and at tributary stations. Although many of the stream gaging stations examined in this study have flow records of up to eight years, flow data prior to 2007 have to be used with caution. To select a representative summer low flow for a stream temperature analysis, the 33 year record at the USGS station at Empire was analyzed to determine 7Q2, and 7Q10 low flows, and monthly median and mean flows. The monthly and 7Q2 flows are representative of typical flow conditions, while the 7Q10 flow is an extreme low flow condition. Analyses were made both with and without the flow contribution of the Empire WWTP (wastewater treatment plant), to asses both pre- and post-2007 conditions. For a July/August composite average, the mean, median, 7Q2, and 7Q10 flows at the USGS station are 68.0, 43.4, 28.6, and 12.5 cfs, respectively, without the WWTP effluent.