EVALUATING THE CAUSES OF HIGH BASEFLOW SUSPENDED SEDIMENT CONCENTRATIONS IN A SMALL SOUTHEASTERN MINNESOTA STREAM

The Root River Watershed (RRW) in southeastern Minnesota is a ~1 million acre drainage basin straddling the boundary between glaciated uplands to the west and the Driftless Area to the east. The western one-third of the watershed is dominated by agriculture and low gradient headwater streams. The remainder of the watershed lies within the Driftless Area and is comprised of a deeply dissected plateau cut into Paleozoic bedrock. Karst springs emanating from Cambrian and Ordovician carbonates feed the numerous high-value, cold water trout streams found throughout RRW.

A critical water quality issue in the RRW is high concentrations of suspended sediment (SSC). Trout stream ecosystems are especially sensitive to high SSCs. Resource managers in the watershed are focused on understanding and assessing the relation between SSC and various land use practices. Since agriculture is the predominant land use in the basin, it receives much of the focus of these investigations.

Our study focuses on understanding the suspended sediment load in two Root River tributaries—Amherst Creek and Forestville Creek. Both streams have been monitored since 2008 and 15 minute time-series turbidity, stage, discharge, and rain data are available. Additionally, grab samples from a variety of baseflow and storm flow events have been collected and analyzed. Both creeks experience periods of high turbidity and SSC in relation to large flow events. However, in contrast to Forestville Creek, Amherst Creek also commonly has high SSC at baseflow discharges.

Turbidity versus SSC rating curves have been established for both streams by correlating the time-series and grab sample data with a polynomial regression analysis. From these data sediment loads can be calculated for both streams. The organic fraction of SSC in Amherst Creek averages about three times that found in Forestville Creek during baseflow conditions. Amherst Creek’s watershed is comprised of significantly more agricultural land use than Forestville Creek, which is predominantly forested and partially within in a state park. We hypothesize that the source of the organics in Amherst Creek is tied to near stream agricultural land use practices.