Temporal variability of shoreline positions and coastal wetlands along lower Green Bay, Oconto and Brown counties, Wisconsin
The positions of shorelines and the areal extent of adjacent coastal wetland tracts in the Great Lakes region have exhibited substantial temporal variability during both prehistoric and historical times. Shoreline migration has resulted in environmental problems such as flooding and the coastal erosion of lakefront property, as well as the destruction of coastal wetland resources. In the Great Lakes region, the main natural cause for changes in shoreline position and adjacent wetland area is lake-level fluctuations, which results from two interactive factors. One factor is the glacio-isostatic rebound of the lake basins that has occurred from the end of the late Wisconsin glaciation to the present. This crustal rebounding has resulted in the slow uplifting of previous lake outlets and warping of lake basins, contributing to changing lake levels and shoreline migration. Historic lake-level gauge records indicate modern differential vertical uplift rates that range from 0.26 ft/century in the southern part of the Great Lakes drainage basin to 1.74 ft/century in the northern part of the basin (Larsen, 1989). The second factor contributing to lake-level fluctuations is climate variability, which causes variations in the amount of regional precipitation and evaporation, storm frequency, runoff, and resulting lake levels. Climate variability can occur over a wide spectrum of time scales, from seasonal variations, to longer-term trends of a few years or decades in duration, to trends lasting hundreds of thousands of years. A combination of both climatic variations and glacio-isostatic rebound has resulting in substantial temporal variability of the Great Lakes shorelines and associated coastal wetland tracts during post-glacial times.