GEOMORPHIC & HYDROLOGIC FACTORS AFFECTING EROSION RATES IN RAVINE TRIBUTARIES TO THE MINNESOTA RIVER

The Minnesota River is the largest contributor of sediment to the Mississippi River in Minnesota. Over the past few decades work has begun to understand how land use change has impacted the system and what best management practices (BMPs) are effective at mitigating sediment delivery. Seven Mile Creek, a tributary to the Minnesota River in South Central Minnesota that is impaired for sediment and nutrients, was chosen for in-depth investigation as part of a US Army Corps of Engineers study of the Minnesota River. This small, agricultural watershed (79.8 km²) provides a case study for the role of ravine systems in the deeply incised Minnesota River basin. Our goal is to understand the sources of sediment and the erosion processes at work in the ravine tributaries of Seven Mile Creek. The physical parameters measured include soil bulk density, soil erodibility, phosphorous, carbon, and those used to calculate BEHI scores. Soil erodibility was measured in the lab using a jet tester device that fires a stream of water at an undisturbed sample collected below the O horizon and approximately 6” in diameter and 6” thick. Blaisdell K values were calculated based on the erosion rates for each soil core. The Blaisdell K values varied widely from 0.198 to 14. These K values reflect the wide range of erodibility in ravine systems. BEHI scores also reflected this variability ranging from 20.6 to 44. These scores correspond to moderate to very high erodibility ratings. The sediment eroding in the ravines appears to be primarily legacy alluvium that was deposited more than 70 years ago (based on tree ring ages) as colluvial terraces and then remobilized as the drainage basin continues to adjust to the base level drop created by Glacial River Warren, as well as to changes in flow resulting from tile drainage and climate change. Future research in the basin will focus on further constraining the sources of sediment and nutrients to the basin through hydrologic monitoring including chemical fingerprinting of water and suspended sediment. Hopefully this research will result in a better understanding of the processes that control ravine erosion and what BMPs can decrease ravine erosion. This can then provide guidance to watershed managers on how to best decrease sediment and nutrient delivery to the Minnesota River, Mississippi River, and beyond.