USING ARTIFICIAL MATS, GRAIN SIZE ANALYSIS, FLY-ASH AND CS-137 DATING TECHNIQUES TO CHARACTERIZE POST-SETTLEMENT ALLUVIUM IN THE UPPER SANGAMON RIVER BASIN IN ALLERTON PARK, MONTICELLO, ILLINOIS

European settlement of the Mid-West United States forced significant changes on the landscape. The implementation of artificial drainage and increases in agricultural activity drastically altered the hydrologic regimes and sediment load of rivers by surface erosion due to the reduction of vegetation. This land-use change created a discernible horizon in the soil profile known as post-settlement alluvium. Determining the thickness of this layer provides the impetus for this study and uses field observations of floodplain sedimentation, fly-ash and Cs-137 dating techniques to determine PSA thickness. The floodplain of Allerton Park is dissected by the Sangamon River and experiences multiple flooding events in a single year. In 2014, a total of five flooding events occurred where deposition was measured using artificial mats. One transect located on the inside of a meander bend saw significant sediment accumulation on mats located close to incipient cutoff and floodways during each flood event with depths ranging from 0.02-4.5mm per event. Grain size distributions shows that the patterns of deposition are far more heterogeneous than fining from the natural levee towards the valley wall. Fly-ash dating estimates the thickness of PSA between 0.6 -1.2m and corresponds with a modern deposition rate determined from artificial mats for single events.