MAPPING FLUVIAL TERRACES TO DETERMINE THE INFLUENCE OF POST-GLACIAL LAKE LEVEL HISTORY ON GRAND PORTAGE CREEK, GRAND PORTAGE, MINNESOTA

Grand Portage National Monument, at Grand Portage, Minnesota, is a narrow eight-and-a-half mile corridor of land that stretches along Grand Portage Creek from Lake Superior in the southeast, to the Pigeon River along the Canadian border in the northwest. The historic Grand Portage Trail follows the valley of Grand Portage Creek through fault lines in diabase dikes. In this study, the interaction of terrace and valley development of the lower 1.2 miles of Grand Portage Creek was evaluated in terms of post-glacial lake level history of Lake Superior. Building on established data (Phillips, 2001) for former lake levels from Early lake Minong through to modern Lake Superior, fluvial terraces along Grand Portage Creek were mapped through the use of remote sensing, in a geographic information system (GIS). In the summer of 2011, field checking of the fluvial terraces was completed by the collection of terrace feature, elevation, vegetation, and soil development observations. The data have been compiled into a GIS-generated thematic map, and offer a glance at the surficial geologic history of the area from 11,000 years B.P. to the present. These data will assist in environmental planning, stream geomorphic assessments, historic site protection, bank stabilization, and archeological preservation by the National Park Service. This project was sponsored by both the Geological Society of America’s Geocorps and the National Park Service Geoscientists in the Parks program.