Abstract

Conversion of native land cover to row crop agriculture and anthropogenically modified hydrology, correlates with increases in peak discharge, annual discharge, sediment and nutrient loads in agricultural land-use dominated watersheds within the Mississippi River basin. This results in environmental issues related to turbidity, eutrophication-hypoxia, loss of biodiversity, natural resource degradation, reduction in tourism, and more. In no place is this more obvious than the ever-growing "Dead Zone" in the Gulf of Mexico. The Minnesota River basin, the largest tributary to the Mississippi River in Minnesota, is a disproportionately large contributor of sediment (~80-90%), nitrogen (~56%), and phosphorus (~45%) to the upper Mississippi River watershed above riverine Lake Pepin. More broadly, despite being 1.34% of the surface area of the Mississippi basin, it contributes 5-7% of the nitrate load to the Gulf of Mexico.

Two agricultural drainage basins, approximately 4 km apart in the Le Sueur River watershed, a Minnesota River tributary, provide a unique opportunity to compare County Ditch 57, recently reengineered to include a suite of structural mitigation practices (surge pond and wetland, two-stage ditch, buffer strips, rate control weir), to a lesser modified Little Beauford Ditch (LBD). The efficacy of CD 57 was evaluated over two years (2016-2017), with monitoring stations bounding each mitigation structure. The surge pond and wetland were efficient reducers of peak discharge, sediment and nitrogen loads, during low magnitude events. However, net increases in discharge, sediment and nutrient loads were still consistently observed. LBD exhibited higher peak discharge, sediment, and total phosphorus than CD 57. CD 57 surpassed LBD in nitrogen. Overall, these results suggest that CD 57 mitigation structures reduce peak discharge, sediment and phosphorous loads/total yields at low-flow events, but are overwhelmed in frequently observed high-magnitude events. This suggests the size/scale of these structures are inefficient for the watershed hydrology. Reduction in peak discharge is promising for limiting peak flows and erosion of bluffs/banks within the knickzone downstream, if similar structures are emplaced throughout the upper watershed. Determining the spatial scale, economic viability, and necessary size of the structures to truly make a broader impact should be the subject of future study.

Advisor

Phillip Larson

Committee Member

Bryce Hoppie

Committee Member

Fei Yuan

Committee Member

Karen Gran

Date of Degree

2018

Language

english

Document Type

Thesis

Degree

Master of Science (MS)

Department

Geography

College

Social and Behavioral Sciences

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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