The Blue Earth River basin, located in the prairie pothole region of northern Iowa and southern Minnesota, is intensively ditched and drained to improve agricultural production. Agricultural drainage systems increase watershed drainage density and can efficiently transport easily leachable agricultural contaminants, such as nitrate, to receiving water bodies. This thesis examines the hydrologic and water quality benefits provided by two restored perennial vegetation-wetland complexes in the Elm Creek subwatershed of the Blue Earth River basin. Flow measurements and water quality samples were collected at the wetland outlets and at drain tiles and surface channels flowing into the wetlands. Four years of flow data showed that the wetlands reduced the magnitude of peak flows to Elm Creek, but that they did not significantly reduce water yield compared to the agricultural watersheds. The restored wetlands decreased nitrate export by 85 percent during the months of April to June, the period when nitrate from agricultural drainage water contributes to formation of the “dead zone” in the Gulf of Mexico. The wetlands did not significantly decrease phosphorus or sediment export to Elm Creek. Water quality benefits attained by trapping phosphorus and sediment from surface runoff were offset by internal phosphorus loading, algal blooms, and sediment resuspension. Empirical modeling of one wetland basin showed that wind speed and wind direction could explain 60 percent of the suspended sediment concentration within the wetland. Active management of water levels and wetland vegetation are presented as strategies to reduce sediment and phosphorus export from restored wetland basins.
University of Minnesota M.S. thesis. May 2012. Major: Natural resources science & Management. Advisor: Kenneth N. Brooks,
Ph.D. 1 computer file (PDF); vii, 109 pages, appendix A.
Fransen, Greg David.
Hydrologic, nutrient, and sediment responses of restored perennial vegetation/wetland complexes in southern Minnesota..
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