GEOMORPHIC HISTORY OF ISLANDS IN GREAT LAKES RIVERMOUTHS AND IMPORTANCE FOR COASTAL HABITAT REHABILITATION AND CLIMATE CHANGE

The geomorphic history of an island complex in the vicinity of the mouth of the Sheboygan River at Lake Michigan was studied by the U.S. Geological Survey and Professor James Knox in 2011-12 to assist the WI Department of Natural Resources in the design and assessment of three large riverine habitat improvement projects associated with the Great Lakes Restoration Initiative. The study included the collection of sediment cores along transect lines that bisected the island complex that were overlaid with digital elevation and bathymetry data. A variety of additional historical and Holocene riverine and lake shoreline data were integrated with the core stratigraphy to build a detailed description of the geomorphic history and diversity of the hydrologic processes over time that formed and shaped the islands. Additional data included the relative elevation differences of the island surfaces, radiocarbon-based age dating, historical changes in channel planform over time from aerial photographs and maps, trends in river flows and Lake Michigan levels. The results were integrated into findings from a previous study on the formation of fluvial terraces and relict Lake Michigan shorelines on the lower Sheboygan River (by a previous student of Professor Knox). The island complex likely began formation about 2,500 years BP after Lake Michigan levels dropped from the Algoma lake phase (179 m) to the modern shoreline of 176 to 177 m. The island complex remains in an area that is commonly at the intersection of riverine flows and backwater effects from fluctuating lake levels and seiche-related currents. A hydraulic model was used to illustrate how the interaction of high and low river flows and lake levels influence aggradation, incision, and bank erosion in the vicinity of the islands. The results from the study illustrate the advantages of combining Holocene histories with changing hydrologic conditions to understand geomorphic processes in important coastal habitats along the Great Lakes.