USE OF NUMERICAL ISOSTATIC DEFORMATION MODELS AND GIS TO PREDICT ICE SHEET HISTORY, LAKE LEVELS AND PALEOHYDROLOGY OF EASTERN WISCONSIN DURING LATE GLACIAL TIMES

Numerical models of glacial isostasy have often been used to predict glacial lake shorelines, but with the availability of high-resolution digital elevation models (DEMs), it is now possible to better constrain the assumptions of these models. When a DEM is allowed to deform through time as predicted by models of isostatic relaxation, an entire hydrologic system with lakes, outlets, shorelines and rivers evolves without the need to incorporate such additional assumptions as outlet positions or history.

Using a model that predicts the deformation of the Earth's solid surface and its geoid since the last glacial maximum given an ice-sheet thickness history, we predict lake levels through time in eastern Wisconsin. Comparison of projected shorelines with field observations will help to constrain the assumed thickness history of the ice sheet as well as the Earth's viscoelasticity. When compared to the observed shorelines from glacial Lake Oshkosh, our initial predictions indicate excessive deformation. This may indicate that (1) the assumed ice-sheet thickness over the Great Lakes region is too great, or (2) the assumption that the ice sheet was in isostatic equilibrium 18,000 years ago is unrealistic. Indeed, field evidence favors a dynamic ice-sheet margin, and so it is unlikely that isostatic equilibrium was actually attained. It is also unlikely that the maximum thickness of the ice-sheet over the Great Lakes region was 2,000 m as assumed in the initial model.

Ongoing work will use an inverse model to calculate ice-sheet thicknesses that are consistent with the observed shorelines. Predictions of proglacial lake volumes through time for glacial Lake Oshkosh will make it possible to estimate peak discharge in the outlet channels during glacial outburst floods. Furthermore, the predicted isostatic deformation affects the paleohydrology of the region and the morphology of the drainage basins, including their size, slope, and mean elevation. To avoid the unrealistic assumption of isostatic equilibrium at 18,000 BP, future calculations will include an ice sheet that advances from 30,000 BP to its maximum extent before retreating. Only when the Earth rheology and ice-sheet thickness history are correct will the entire paleohydrologic system, evolving through time, match observations.