Sediment budgets indicate Pleistocene base level fall drives erosion in Minnesota's greater Blue Earth River basin

Abstract

Minnesota River (MNR) tributaries are some of the most turbid in the state; many are impaired for turbidity under the Clean Water Act. Suspended sediment affects ecology and economics from headwater streams to Lake Pepin, where much of it is deposited. This project created sediment budgets for the greater Blue Earth River basin (GBERB), a group of MNR tributaries with some of the highest sediment loads. A sediment budget is a way to understand the movement of sediment though a watershed that can help landowners, land managers and other interested parties allocate resources to effectively reduce sediment loads. Our budgets use historic aerial photos and lidar-derived digital elevation models to delineate source extents and measure bluff and channel erosion rates in ArcGIS; these data were combined with upland and ravine erosion rates measured in the Le Sueur watershed. We explored sediment budget sensitivity to adjustments for sediment storage, bluff vegetation state, sedimentology, erosion rate extrapolation methods and higher-precision bluff extent delineations. Tributaries of the Minnesota River are adjusting to a profound (70m) base level fall at the end of the Pleistocene. About half of the GBERB sediment load comes from reaches below knickpoints where response to base level fall drives erosion of near-channel features like bluffs. Budgets are not sensitive to bluff erosion rate extrapolation techniques and we found no statistically significant correlations between decadal bluff retreat rates and parameters such as bluff vegetative cover, slope, size, aspect, sediment texture or stream power. There is little in-stream sediment storage in the GBERB: accommodation space primarily occurs on floodplains and in lakes, but these features are scarce due to base level fall and agricultural practices. Surficial sediment in the GBERB is composed primarily of homogeneous glacial tills and load estimates have little sensitivity to adjustments for the different bulk density and texture of glaciolacustrine and glaciofluvial sediments. It is important to construct an accurate inventory of bluff extents: A poorly-managed but plausible inventory increased the sediment budget by about 15%. These results will be useful in constructing sediment budgets for other MNR tributaries and in managing the GBERB. Shapefiles and data used in this project are available through the University of Minnesota Digital Conservancy.