Adjustment of prairie pothole streams to land-use, drainage and climate changes and consequences for turbidity impairment

Changes in land use and drainage have contributed to channel adjustment in small-order to medium-order streams in the prairie pothole region of south-west Minnesota. Although conversion from prairie to agriculture occurred a century ago, recent decades have seen increased subsurface tile drainage, annual row crop coverage and channel modifications, particularly at road crossings such that channel adjustment is ongoing. Channel evolution in Elm and Center Creeks, two fourth-order streams in the Blue Earth River basin, was studied to understand relationships between changes in channel morphology and suspended sediment concentrations. The construction of drainage ditches and expanded subsurface tiling has connected isolated basins to stream channels, effectively increasing drainage areas of Elm and Center Creeks by 15-20%. Sinuosity has been reduced by grading and drainage of first-order sloughs, channel straightening at road crossings and natural cut-offs and agricultural ditching that have shortened Elm Creek by 15% between 1938 and 2003. Stream cross-sectional area was enlarged in response to the land-use and drainage changes. In the headwaters, public ditches are wider than historic channels and entrenched by berms. Unchannelized headwater and upper mainstem portions of Elm Creek are also highly entrenched (up to 1.07 meters below the pre-channelization bed elevation with a bank height ratio>1.5) but have not widened substantially. In contrast, the lower main channel has widened by an average of 68%. These channel adjustments contribute to the suspended sediment load and violations of Minnesota's turbidity and Index of Biotic Integrity standards. The watershed has a low sediment delivery ratio because it is a flat, poorly connected landscape and likely delivers less sediment to the Minnesota River than steeper rivers downstream, such as the Blue Earth River. Entrenchment and increased sediment transport capacity in the lower reaches of the river have lead to increased sediment delivery to the downstream Blue Earth and Minnesota rivers. Understanding geomorphic changes will be important for addressing water-quality impairments in the region.