PREDICTED LATE GLACIAL AND POST GLACIAL RIVER SYSTEMS AND GROUNDWATER FLOW REGIME IN THE GREAT LAKES REGION

As the Laurentide Ice sheet retreated from the Great Lakes region the hydrologic processes were impacted tremendously. Not only did the lakes change in extent and volume as the ice uncovered outlets and glacial isostatic adjustment progressed, but rivers changed their course, catastrophic glacial burst floods occurred and groundwater flow changed dramatically. Coupling of a model of isostatic adjustment with a GIS hydrology model and a groundwater flow model provides predictions of the movement of water during the past 20,000 years.

Ancient topography can be predicted by deforming the present digital elevation model by an amount equal to the isostatic relaxation as predicted by a viscoelastic deformation model. Once this paleo-topography is known GIS methods are used to determine drainage basins and river courses through time. Although the most dramatic changes in river directions occurred as the ice was retreating, glacial isostatic adjustment forced river changes because of tilting of the landscape even after ice was no longer present in the Great Lakes region. A detailed look at central Michigan indicates river channel shifts of more than 3 km and stream capture events that change river drainages completely forced only by glacial isostatic adjustment.

Tilting of the Great Lakes also affected the volume of the lakes such that even during the past 1000 years decanting caused lake volumes to change by more than 3 km3 independent of meteorological variation. The Hydrologic Engineering Center HEC-RAS model predicts flow velocity and stage heights in rivers given channel geometry. We use this model and the reconstructed glacial and post glacial topography to evaluate the outlet discharge when lake levels are at the Glenwood, Calumet and Nipissing stages. Predicted discharges are high but not implausible.

Furthermore an analysis of groundwater flow in eastern Wisconsin shows that the dramatic changes in head as ice sheets retreated, lake level elevations fluctuated and tilting from glacial isostatic processes forced huge changes in groundwater regime. Predictions of groundwater flow at four times indicate the direction of groundwater flow reversed in the region between glacial times and present.