LAKE-LEVEL INFLUENCES ON SHORELINE BEHAVIOR RECOGNIZED IN GPR DATA FROM GRAND MARAIS HEADLAND, LAKE SUPERIOR, MICHIGAN

The subsurface architecture of the Grand Marais headland beach along southern Lake Superior, MI, was studied using ground penetrating radar (GPR) and geographic information system (GIS) technologies to determine how lake-level changes have influenced shoreline behavior. Jetties constructed in 1883 to protect the harbor inlet have served as a blockade for sediments transported eastward along the shore by prevailing longshore currents. The resulting buildup of sediment along the west side of the jetty has resulted in near-continuous beach growth at an average rate of 100,000 m³ per year. Aerial photographic reconstructions show sediment build-up to fluctuate over the decades. Geophysical data was investigated to examine subsurface features and determine whether sediment architectures are influenced by lake level changes.

The 250 MHz GPR mapped lakeward-sloping sediment units, interpreted as former foreshore deposits, topped by horizontal units, the product of wind processes. Data reveal changes to this basic architecture due to onlap, truncation, and erosion of surfaces. This is caused by influences of different water levels on sedimentation throughout the decades. A beach chronology based on aerial photos, derived from GIS technologies, show that patterns of onlap mapped in GPR coincide with lake-level rise.

Emplacement of the jetty has allowed for preservation of shoreline architectures not accessible elsewhere along the shore. Patterns detected from the subsurface data at Grand Marais headland provide useful information on how lake levels influence shoreline dynamics. Additional data collection, particularly seasonal and year-to-year surveys would provide greater knowledge of how short-lived processes factor into shoreline growth.