Rivers create beautiful patterns and provide drinking water to millions. However an alarming number of rivers in the US and globally are threatened by excess sediment and nutrients. Agricultural rivers draining erodible soils are particularly vulnerable. Rivers of southern Minnesota provide a unique opportunity to study water and sediment dynamics in a naturally vulnerable system. Sediment reduction strategies are needed to ensure biological integrity and adequate water quality. Here, I address the questions: 1) have climate, land use practices, or both affected streamflows in Midwest agricultural rivers?, 2) which streamflows set the rate of river bluff erosion?, and 3) how do sediment supply and transport influence the form and behavior of the lower Minnesota River? Chapter 2 demonstrates, in three agricultural basins, that artificial drainage practices have decreased soil moisture, contributing to increases in streamflow. Chapter 3 quantifies river bluff erosion and identifies erosion by streamflows as the dominant erosion process. Erosion by common floods accomplishes the most cumulative bluff erosion. Bluff erosion contributes sediment to the Minnesota River. Chapter 4 shows how this coarse sediment influences the form and behavior of the Minnesota River. Therefore if flows were reduced, bluff erosion would slow, and the supply sediment to the Minnesota would slow, leading to less streambank erosion. Since streamflows have been increased by agricultural drainage practices, water retention solutions are needed to reduce high flows.

Thesis (M.S.)--Wisconsin. / Includes bibliographical references (leaves 150-161).

This study focuses on quantifying explicitly the sediment budget of deeply incised ravines in the lower Le Sueur River watershed, in southern Minnesota. High-rate-gully-erosion equations along with the Universal Soil Loss Equation (USLE) were implemented in a numerical modeling approach that is based on a time-integration of the sediment balance equations. The model estimates the rates of ravine width and depth change and the amount of sediment periodically flushing from the ravines. Components of the sediment budget of the ravines were simulated with the model and results suggest that the ravine walls are the major sediment source in the ravines. A sensitivity analysis revealed that the erodibility coefficients of the gully bed and wall, the local slope angle and the Manning’s coefficient are the key parameters controlling the rate of sediment production. Recommendations to guide further monitoring efforts in the watershed and increased detail modeling approaches are highlighted as a result of this modeling effort.