A GIS APPROACH TO MAPPING AND ANALYZING FORMER SHORELINES WITHIN THE LAKE SUPERIOR BASIN

The late Quaternary history of lakes within the Lake Superior basin is based primarily on field mapping of raised shoreline features (beach ridges, wave-cut scarps, and deltas), but existing studies are regional in scope, and elevation observations are discontinuous. Using 10-m digital elevation models (DEM) and a GIS, shoreline features can be traced over great distances and their elevations plotted as continuous data to identify distinct strandlines that correspond to former lake levels. Continuous strandline elevation data allow an accurate determination of strandline slope, which is a function of age (steeper strandline slopes are created by a longer period of post-glacial rebound). Analyses have been completed for lake levels associated with glacial Lake Duluth in the western Lake Superior basin. There are four mappable lake levels that have all been named previously. From highest to lowest they are the Duluth, Highbridge, Moquah, and Washburn levels. Shoreline features associated with the Duluth level are the most conspicuous, but the strandline begins to splay to the northeast and becomes less apparent. Shoreline features associated with the Washburn level are nearly as well developed as the Duluth level, but the Highbridge and Moquah strandlines are only apparent in DEMs along the southern shore in Wisconsin and Michigan. Unlike previous reports, none of these strandlines have hinge lines beyond which no deformation exists, but instead have slopes that steadily increase to the northeast. There is very little difference in curvature between the highest and lowest strandlines, suggesting they all date to about the same age (although the highest lake level must be the oldest). Radiocarbon dated sediments from small basins isolated after lake level drawdown suggest that the Duluth level is slightly older than around 10,700 cal (9400 ¹⁴C) yr BP.