SEDIMENT FINGERPRINTING FOR SOURCES AND TRANSPORT PATHWAYS IN THE ROOT RIVER

The movement of sediment from uplands, to the stream and eventually out of the watershed is a complex process. Erodibility of the soils, distance to the stream, amount and intensity of precipitation, hydrology of the watershed, sediment transport capacity of the stream and the channel-floodplain exchange are all factors in sediment routing. Previous work done by Trimble (1999) in Coon Creek, Wisconsin, found that large amounts of eroded sediment did not simply flush out of the system into the Mississippi River, but rather were stored for decades within the watershed demonstrating that the amount of sediment transported out of the system is not always representative of erosion occurring within the watershed. We hypothesize that a similar story has unfolded in the Root River watershed, southeastern Minnesota, namely that large amounts of sediment previously eroded from agricultural fields and hillsides has been stored in the floodplains, and the river has recently begun to erode floodplains, actively transporting the stored sediment through the river network. Currently, there are seven reaches of the Root River network impaired under EPA standards for turbidity. The impairment of these reaches forms the basis for the following questions: Where is the fine sediment coming from? How does it move through the watershed? How are fine sediment sources changing with different events and in different parts of the watershed? We describe how, through the use of geochemical tracers (Beryllium-10, Lead-210 and Cesieum-137), we have determined the “fingerprint” of the sources of sediment. Using the ratio of the three tracers measured in suspended sediment samples collected during storm events, we have constrained the proportion of sediment derived from upland versus non-upland sediment sources as well as the significance of channel/floodplain exchange. Early results indicate that upland sources are contributing more sediment than anticipated.