Stream Morphological Response to Climate and Land-Use in the Minnesota River Basin

The Minnesota River drains one-fifth of the state of Minnesota and is a major tributary to the upper Mississippi River. Intensive row crop agriculture is the primary land use in the basin. To develop higher crop yields, both surface and subsurface drainage were applied to the landscape over the 20^th century. A 1989–1993 watershed assessment study suggested a linkage between degraded water quality and land-use changes, particularly agricultural drainage. Twelve USGS gaging stations with extensive flow records located in the Minnesota River basin were analyzed for changes in peak flow for frequent events. Twenty-five USGS gaging stations across the Minnesota River basin representing varying drainage areas were analyzed for field morphological features. Stream reaches were classified using Schumm's channel evolution model and Rosgen's stream classification system. Results suggest that wetland and prairie-lake conversions to cropland have increased runoff contributing drainage areas. These changes, along with changes in climate, are reflected by increased flows in frequent events (1.01-year, 1.5-year, 2-year recurrence intervals) throughout the century. Field evidence of channel incision was observed in the Yellow Medicine, Chippewa, Cottonwood, Blue Earth and Rush Rivers. Increases in the bankfull, channel-forming discharge have resulted in channel degradation, which disconnects the channel from the active floodplain. The resultant loss of the active floodplain has destabilized riparian vegetation, habitat and the riparian corridor's ability to buffer environmental stress. Subsequent channel adjustments following incision have resulted in land loss and increased sediment loads in the unstable channels. Some of the increased sediment loads were deposited in downstream, widened reaches, but silts and clays were transported further downstream as an increased suspended-sediment load.