RESPONSE AND RECOVERY OF UPPER MIDWEST FLUVIAL SYSTEMS TO AGRICULTURAL DISTURBANCES

Agricultural land use typically accelerates rates and magnitudes of natural erosion and sedimentation because a balance between climate and vegetation is disrupted. The hilly lands of southwestern Wisconsin were a mosaic of prairie and forest about 200 years ago but within a period of about 50 years were transformed to a largely agricultural landscape of cultivated fields and pastureland. Accelerated surface runoff and severe soil erosion, followed by destabilization of channel systems, characterized the landscape by the 1870s and continued until the early 1950s when better land conservation and improved cropping and grazing practices became significant. Responses and recoveries to changing land use have been both complex and slow due to remobilization of stored agricultural-related sediment and to positive feedback effects associated with historical development of meander belts that facilitate accelerated downstream flood conveyance. The close linkage between upland land cover and downstream fluvial activity indicates that efforts by some to restore channels to pre-agriculture morphology are likely to be problematic because the downstream design channels are not in equilibrium with the upland landscape hydrology. Landscape sensitivity to climatic events was greatly increased in response to agricultural land use. Average magnitudes of bankfull stage floods increased between 200-400% of those for the pre-agriculture natural prairie and forest landscape while Holocene floods experienced average variations of ±20% in response to episodes of climate change. Decadal average overbank sedimentation rates commonly ranged between 1-2 cm per year during the agricultural period but the long-term Holocene rates averaged only 0.02 cm per year. Global-circulation-model results indicate global warming will increase the frequency and magnitude of extreme hydrologic events in the upper Midwest if global warming continues. The region's river systems may therefore experience prolonged recovery with global warming due to agriculturally accelerated sensitivity of the landscape to climatic events. This research was supported by the National Science Foundation and by a University of Wisconsin Evjue-Bascom Award.