Lake Michigan is the only Great Lake lying entirely within the United States, spanning about 14.3 million acres with shoreline in Michigan, Wisconsin, Illinois, and Indiana. It connects to Lake Huron at the Straits of Mackinac and forms one hydrologic lake with Huron; the U.S. Army Corps of Engineers and Canadian partners publish coordinated lake-wide water-level information for the Michigan–Huron system.

With roughly 1,640 miles of shoreline, an average depth near 279 feet, and a maximum depth of about 925 feet, Michigan includes deep open basins, dune coasts, and major embayments such as Green Bay and Grand Traverse Bay. Wind-driven setup and seiches can raise or lower local water levels by feet within hours, and coastal morphology ranges from sandy beaches to rocky bluffs and industrial harbors.

The lake supports commercial ports, beaches, boating, and major recreational and commercial fisheries for salmon, lake trout, yellow perch, and other species. Michigan waters are managed with guidance from the Michigan Department of Natural Resources, while Wisconsin, Illinois, and Indiana agencies oversee their respective shorelines and fisheries.

Lake Michigan does not use reservoir-style full, winter, or flood pools. Chart datum (Low Water Datum) for Michigan–Huron is 577.5 feet IGLD85, and lake-wide surface elevations are reported in the International Great Lakes Datum of 1985 (IGLD85)—not as a simple MSL reservoir pool. Levels respond to basin water supply, connecting-channel flows, and seasonal climate, so monitoring current IGLD85 elevations and marine forecasts is essential for navigation, harbors, and shoreline use.

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