Soil Erosion and Hydrologic Processes in Ravines within Agricultural Landscapes of Southern Minnesota.

A study was undertaken in 2013 to better understand erosion and hydrologic processes in steep intermittent ravines within the Seven-Mile Creek watershed as part of the U.S. Army Corps of Engineers Interagency study of the Minnesota River basin.  Some of the research questions addressed in the study included, “How do soil and geologic properties influence erosion rates in ravines”?  What flow levels generate erosion ? and, "What is the role of vegetation type and cover in resisting erosion"? Methods included analysis of ravine soil and geologic layers for particle size, bulk density and organic matter at 23 sites.  The soil erodibility coefficient (k) and the critical shear stress were measured with a jet tester in the laboratory with a 15cm core from the ravine banks.  Plant community composition, cover and floristic quality were also measured along the channel. Stream flow monitoring was undertaken in 2014-2015 in two tributary ravines that were previously monitored.   Modeling with the Bank Stability and Toe Erosion Model (BSTEM) will be used to assess the role of soil properties, plant root traits and groundwater inputs on erosion rates.  The k values ranged widely (0.7 – 138 cm³/N-s). Dense glacial tills (> 1.7 g/cm³) typically had low k and high critical shear stress while alluvial soils on lower terraces and stream valleys were more highly erodible. Plant diversity and floristic quality was high in the ravines and low in tributary ditches. Plant coverage was greatest in areas with dense herbaceous vegetation underlying the forest.  Streamflow duration was 1% of the 500-day record, with only one major ravine erosion event in 2014.  Future work will include estimation of sediment loading from channel-resurveys, LiDAR analysis, bank erosion indices and sediment delivery calculations.  The data is being used to prioritize watershed and ravine management activities to reduce sediment loading to the Minnesota River.