THE IMPACTS OF PAST GLACIATION ON THE RESPONSE TO DEVELOPMENT ACROSS THE UPPER MIDWEST, UNITED STATES

Land use across the Midwest United States underwent a dramatic shift toward intensive agriculture beginning in the mid-nineteenth century. This change led to landscape responses ranging from soil erosion and gullying to floodplain aggradation and incision. We hypothesize that the spatial variability in the response to development was largely controlled by the variability in the timing and nature of glacial geomorphic processes across the region. Using soil-geomorphic mapping together with fly-ash from coal combustion as a sedimentary marker for historic alluvium, we compare the type and magnitude of response to development on a pre-Illinoian surface (Clear Creek, Iowa) to the response of the Wisconsin-age Upper Sangamon River in central Illinois. At Clear Creek, post-settlement floodplain sedimentation rates exceed those observed in the Sangamon River. Additionally, in the lowermost reaches of Clear Creek, post-settlement alluvium has been incised by the modern river whereas there is no evidence for significant incision of the Upper Sangamon since agricultural development. These results are compared with previous research into historic geomorphic change in the Driftless Zone of southwestern Wisconsin and the Minnesota River Valley to further assess the impact of glacial geomorphology on recent landscape change. Preliminary results indicate that the time since most recent glaciation, the relief of glacial landforms, and the occurrence of catastrophic flooding associated with glacial lakes strongly influenced the evolution of Midwestern landscapes over the last two centuries. While much of this region is currently under a strikingly uniform corn and soy monoculture, the legacy of contrasting glacial history is observable in the initial response to agricultural development and is likely to continue to influence geomorphic change into the future.