FEASIBILITY STUDY FOR MAPPING DISCONTINUOUS STRANDLINES IN THE GLACIAL LAKE AGASSIZ BASIN

Numerous researchers have reconstructed water planes of glacial Lake Agassiz by mapping discontinuous strandlines throughout the basin. This attempt using 7.5' topographic maps, 1/3 arc second digital elevation models (DEMs) and satellite imagery, focuses on the southeastern basin in Minnesota from 46°30' to 48°07'30” N latitude along a ~240 km transect between 298–335m (980 to 1100 ft) elevation. The intent was to determine the feasibility of tracing water planes with confidence, and if so, to connect the known Campbell, Tintah, Upham, Norcross, and Herman strandlines northward. To limit bias, the initial mapping was conducted with no knowledge of the elevations or locations of the named strandlines. Water planes were identified from escarpments (scarps), bars, gravel pits/deposits and spits. The base of the scarp was chosen as the water plane elevation when spits where not present, as we use spits as the best indicators of paleo water planes. Bars are considered minimum elevations of water planes, and longer more continuous strandlines were favored over smaller, discontinuous ones. Elevation assignments were made to the nearest contour interval (5 or 10 ft) for each apparent water plane assigned to each 7.5' topographic sheet. Six water planes emerged from this study site, with four of them traced to the towns for which they are named, and new strandlines observed branching to the north. All strandlines mapped exhibit a linear trend on an elevation versus distance plot, increasing in elevation northwards; indicating that the isostatic rebound in this study area is linear. This attempt has shown that it is feasible to determine water planes from strandlines, across some distance, with some degree of confidence. However, the distance one can predict water planes with certainty, remains unknown.