Lake Michigan is the third largest of North America's Great Lakes, and is the only one entirely within the United States, while Canada shares sovereignty over the other four lakes. The lake is therefore the largest freshwater lake in the United States. It extends about 520 km from north to south, and measures about 100 km in its maximum width, with long stretches of scenic shoreline, beaches, bays and inlets. The lake water flows out to L. Huron through Mackinac Straights.

Since the middle of the last century, urbanization and industrialization have progressed rapidly along the lake's southern shore, which is now one of the most highly industrialized areas in the United States. The lake water along the southern coast has been seriously eutrophicated since the early 1970's. To counter this trend, a number of laws have been legislated and wastewater treatment plants constructed. The use of synthetic detergents containing phosphorus was thereby prohibited in the lake's watershed. The wastewater, that had once entered the lake, was diverted to the Mississippi River. As the result, the quality of the lake water is now recovering gradually.