Climate and Landform Impacts on Flow, Sediment, Phosphorus and Nitrogen Losses from the Minnesota River Basin

Dr. Satish Gupta

Professor Emeritus, Dept of Soil, Water, and Climate, University of Minnesota

Abstract

The presentation will summarize over 20 years of research by the author, his collaborators and their students on sources and causes of environmental pollution in the Minnesota River and its tributaries. Hopefully, this presentation will correct some of the misperceptions about these environmental problems and lead to better discussions on effective mitigation strategies. The Minnesota River basin is a major agricultural producing area in the state. It is relatively flat with large numbers of shallow depressions left after the last glaciation. Because of tight glacial tills, tile drainage is necessary for producing healthy profitable crops. On average, 75% of the water in streams is the water that infiltrates through the landscape. This infiltrating water picks up soluble nitrogen in its path to drain tile leading to elevated nitrate concentrations in river waters. Most of the tributaries are also lined with tall river banks that developed naturally after the last glaciation. These banks are continuously sloughing and contributing sediments to the tributaries and in turn to the Minnesota River. As much as 79% of the sediments in tributaries are from river banks. Among the major mechanisms causing bank sloughing are seepage, freezing and thawing, wetting and drying, and fluvial erosion. Fine particles in these bank materials strongly adsorb phosphorus as the sediments tumble downstream. Although there is some leaching of soluble phosphorus from agricultural lands through drain tiles, WWTPs and legacy P are still a significant source of soluble phosphorus in river waters. Using the above concepts, we showed that phosphorus in Lake Pepin sediments was from industrial and domestic waste that was dumped in the Minnesota and the Mississippi Rivers. Because of low relief and depressional nature of landscape, surface sediments leaving agricultural lands are limited. Changing climate is another factor that is exacerbating environmental problems in the Minnesota River Basin. Wet climate is increasing river flows as well as bank sloughing and N and P leaching. Recent increased river flows are not due to tile drainage as has been promoted by agencies and popular media but are due to excess water and lower evapotranspiration during wet years. Higher sediment loads have also been blamed for the decimation of native mussels in the river system. This presentation will show that such decimation was the result of our dumping waste in rivers as well as harvesting of mussels for the button industry.