A GEOGRAPHIC INFORMATION SYSTEM RECONSTRUCTION OF GLACIAL LAKE OSHKOSH AND ANCIENT HYDROLOGY OF EASTERN WISCONSIN FROM MODELS OF GLACIAL ISOSTATIC ADJUSTMENT

The extent of glacial Lake Oshkosh in eastern Wisconsin during the most recent glaciation was controlled by the location of the ice margin and changes in outlet elevations due to glacial isostatic adjustment. During the retreat of the Green Bay Lobe, a series of progressively lower outlets was uncovered, resulting in dramatic drainage events that rapidly decreased the lake’s volume. Outlet elevations differentially changed over this period as a result of the isostatic depression and rebound of the region caused by advance and retreat of the ice margin.

We use a numerical model of glacial isostatic adjustment on a viscoelastic earth and an ice-load model that best fits sea-level data to calculate the time-dependent deformation in the region. The models determined that more than 200 meters of differential tilt existed across the extent of the lake. Subtracting the modeled deformation of the region from a digital elevation model of present elevation yields the ancient topography (paleo-topography) that can be used in a geographic information system to determine the past hydrology (paleo-hydrology) of eastern Wisconsin. Drainage basins, integrated stream systems, and lakes can be determined on the paleo-topographic surface for any time in the past.

Preliminary results indicate that 16,000 years ago glacial Lake Oshkosh was approximately 150 km long and 70 km wide with a volume of 1,870 km³. Much of that water would have drained rapidly when lower outlets became ice-free. The GIS can also be used to simulate shoreline locations at different stages of Lake Oshkosh. These locations can be superimposed upon U.S. Geological Survey topographic maps and used for field checking the predictions and, later, in focusing surficial mapping.