Freshwater ridge-and-swale ecosystems are globally rare and found only in parts of the Great Lakes of North America. They were formed as a result of the gradual retreat of beaches due to falling water levels and post-glacial rebound. The swales and the adjoining lake or river commonly form a single hydrological unit, so that rising or falling lake levels will cause water levels in the swales to rise or fall as well. Most ridge-and-swale landscapes have been destroyed as a result of the massive industrial development found in many former ridge-and-swale areas, such as the Calumet Region of Northwest Indiana. However, some extensive dune-and-swale complexes persist in Michigan, such as at the Michigan Wilderness State Park. The remaining dune-and-swale complexes along the Great Lakes are often home to extremely rare plants and animals, such as the endangered Karner Blue butterfly.
