An assessment of land use impacts on channel morphology in a western Minnesota watershed.

摘要

This research is a comprehensive investigation encompassing land use changes in an agricultural watershed and corresponding changes to the rainfall-runoff relationship and stream channel morphology. The Lac qui Parle (LqP) watershed is one of eleven major watersheds within the Minnesota River Basin (MRB). Agriculture is the dominant land use within the MRB occurring on more than 90-percent of the landscape. This research investigates the spatial and temporal changes in channel morphology and land use between 1965/66 and 2002/03. Historical data were obtained from the US Army Corps of Engineers for the South Fork Lac qui Parle (SF LqP) River. Sixty-five cross-section sites were re-surveyed and evaluated. Current channel morphology was assessed through a second year of data collection. Additional data were collected and analyzed for crop history, riparian vegetation, agricultural drainage, annual discharge, annual peak discharge, and monthly and annual precipitation.Results indicate crop diversity within the SF LqP River has diminished and is currently dominated by corn and soybean. Surface and subsurface drainage of agricultural lands was documented on 37-percent of the sub-watershed area evaluated. Analysis of the discharge and precipitation records indicates an increase in the Q/P ratio and average annual runoff volume post-1960.The analysis indicates land use changes within the SF LqP watershed have impacted the channel morphology of the SF LqP River post-1965/66. Significant changes in channel cross-sectional area were noted in the Middle and Upper watersheds. Results corresponds to a channel enlargement ratio (CER) of 1.02--1.30. Changes in cultivation practices and drainage activities correspond with higher CER of 2.32--2.6. Similar increases were noted for peak discharge (1.30--1.35). CERs match values developed for storm sewered streets in urbanized areas. All changes were significant at the 95-% confidence level.A separate investigation evaluated the use of natural channel design for agricultural ditches. Results indicate natural channel design provides more efficient sediment transport, increased channel diversity/complexity, and may reduce channel maintenance costs.Key Words: Lac qui Parle River, channel morphology, Rosgen classification, channel evolution model, channel enlargement ratio, agricultural drainage, bankfull, grazing, annual discharge, peak discharge, climate change, Q/P ratio.