NUMERICAL MODELS OF WISCONSIN HYDROLOGY SINCE THE LAST GLACIAL MAXIMUM: LAKES, FLOODS, GROUNDWATER AND RIVER CHANNELS

As the Laurentide Ice Sheet retreated across Wisconsin proglacial Lake Oshkosh formed south of its margin. The lake configuration changed dramatically as the ice sheet uncovered 5 successively lower outlets and as glacial-isostatic processes affected elevations of those outlets. Numerical predictions indicate each outlet uplifted more than 100 m and then subsided approximately 30 m. With these numerical predictions of isostatic adjustment, high-resolution digital elevation models (DEMs) and GIS methods, it is possible to reconstruct Wisconsin paleo-topography and the late-glacial and post-glacial evolution of hydrology in the region. Predictions include the bathymetry of Lake Oshkosh, outburst flooding magnitudes, groundwater flow and river channel changes. At its maximum extent Lake Oshkosh covered 6600 km² with a volume of 111 km³. When we used the HEC-RAS model to simulate glacial outburst floods, flow velocities up to 9 m/s and peak discharge of 140,000 m³/s were predicted, which would drain the lake in 10 days and transport boulders up to 3 m in diameter. Such floods resulted in erosional channels now occupied by underfit streams. Coupling the isostasy/GIS predictions with a GIS hydrology model of surface flow results in predicted river channel changes of more than 3 km even during post-glacial times. These changes are therefore caused solely by glacial-isostatic tilting of the landscape. The dramatically changing hydraulic heads caused by i) lake level changes of Lake Oshkosh and adjacent Lake Michigan, ii) isostatic tilting and iii) ice sheet thickness fluctuations, forced huge changes in the groundwater regime. Predictions indicate that groundwater flow direction, discharge and recharge regions reversed between glacial times and the present. These Lake Oshkosh predictions served as a test area for hydrology predictions over the entire Great Lakes region.