Shifting Shorelines Modeling the 20,000-year history of the Great Lakes By James A. Clark and Kevin M. Befus, Wheaton College, Wheaton, Illinois Shorelines of the North American Great Lakes during the past 20,000 years were modeled using ArcGIS predictions of earth deformation and the hydrology tools in the ArcGIS Spatial Analyst extension. During historical times, water levels of the modern Great Lakes have fluctuated by more than a meter. This is largely caused by changing weather patterns and the associated rates of evaporation and discharge of rivers and groundwater entering and leaving the lakes. However, when one considers changes in the Great Lakes over the past 20,000 years, the level of the lakes has changed even more dramatically—by more than 100 meters. These changes were caused not only by climate effects, but (more importantly) because the great ice sheet that covered half of the North American continent 20,000 years ago blocked the modern outlets of the Great Lakes, other outlets had to be used. The earth was deformed under the huge weight of the ice sheet, a further complicating factor. The Great Lakes region is between 41 and 49 degrees north latitude, and the ice sheet extended south to about 40 degrees. As the ice sheet retreated northward, outlets became ice free and controlled the level of the lakes. But these outlets were also changing in elevation because the earth was slowly rising to the undeformed position it held in the preIce Age. Creating Paleo-DEMs Based on geophysical theory, it is possible to predict how the viscous mantle of the earth moves under surface loads. In this inquiry, the surface loads included changes in ice sheet thickness over Hudson Bay that were probably in excess of 3,000 meters and accompanied by changes in ocean loads as the melting ice sheets caused the ocean level to rise by approximately 100 meters. Taking all time-dependent loads and the viscosity structure of the mantle into account, numerical geophysical models can predict the vertical deformation anywhere on the earth at any time during the past 20,000 years. As ice advanced over the Great Lakes region, the earth surface under the ice subsided, while the region beyond the ice bulged upward. In general, as the ice retreated, the process occurred in reverse. The land rose under the melting ice but subsided in regions near the ice sheet margin. 24 ArcUser Summer 2009 This diagram shows the deformation of the earth's surface along a transect from Hudson Bay to the Gulf of Mexico. The transect is shown as a red line on the locator map. Curved lines are labeled with the age in years before present. Where the earth was deformed the most by the thickest portion of the ice sheet, the land rises gradually. Near the margin of the ice sheet, the land first rises and then subsides. Contours of deformation at a time 13,000 years ago relative to the present (shown in meters). In the northern part of the map, land was more than 500 meters lower than present-day elevations. South of the zero contour line, land was actually higher than current elevations. The position of the ice sheet and a shaded relief PaleoDEM are included. www.esri.com