GEOPHYSICAL PREDICTIONS OF THE HYDROLOGIC HISTORY OF THE GREAT LAKES

Over the last century geological studies of the ancestral Great Lakes have confirmed that the large surface load of the Laurentide ice sheet deformed the region causing tilting of ancient lake shorelines. We developed a method, utilizing a numerical model of glacial isostatic adjustment combined with GIS and high resolution digital elevation models to predict the paleo-topography at 1000-year intervals. GIS was then used to recreate the 30,000-year paleohydrology of the Great Lakes. Predictions include the extent of late glacial, postglacial and Holocene lakes and their associated outlets and bathymetries. This predicted history of the Great Lakes is similar to that obtained from a century of detailed field studies but our method uses only the present digital elevation model, a prescribed ice sheet chronology and an assumed earth viscoelastic rheology. GPS data support our conclusions that the Great Lakes, south of a “hinge line”, has never been stable. The geometry of outlet channels and the predicted water level and lake volumes permits an estimation of the discharge that likely occurred during outburst floods. Ancient lake bathymetry predictions provide an estimate of water loads associated with each lake and these loads also impact the isostatic adjustment process. Using the predicted paleo-topographic surface it was possible to predict where river drainage changes were caused by the isostatic process. GIS hydrology modeling packages were used to predict the drainage patterns during the past 10,000 years and regions where the patterns alter through time were highlighted for further study in the field. Finally the thickness of the ice sheet over the Great Lakes region was estimated from comparison of the observed deformation to predictions and a relatively thin ice sheet is suggested by the predictions.