Lake Michigan, 22,178 sq mi (57,441 sq km), 307 mi (494 km) long and 30 to 120 mi (48–193 km) wide, bordered by Mich., Ind., Ill., and Wis.; third largest of the Great Lakes and the only one entirely within the United States. Its surface is 581 ft (177 m) above sea level, and the lake is 923 ft (281 m) deep.
The Straits of Mackinac, its only natural outlet, connect the lake with Lake Huron to the northeast; the Illinois Waterway links Lake Michigan with the Mississippi River and Gulf of Mexico. Many islands are found in the northern part of the lake; the northern shorelines are indented, with Green Bay and Grand Traverse Bay the largest bays. The southern part of Lake Michigan has a regular shoreline necessitating the building of artificial harbors such as the Calumet Harbor, NE Ill. The Muskegon, Grand, Kalamazoo, Fox, and Menominee are the chief rivers flowing into Lake Michigan; the lake's current tends to clog the mouths of the rivers with sand. The Chicago River formerly flowed into the lake, but its course was reversed in 1900.
The forested northern region of Lake Michigan is generally sparsely populated. The southern portion, located near the heart of the Midwest, is industrially important; the Gary-Chicago-Milwaukee urbanized area extends along the southwestern shore. Michigan City, Gary, Chicago, Racine, Milwaukee, and Escanaba are the major lakeside cities. Such urban and industrial concentration has led to growing pollution problems associated with the lake's waters.
Prevailing westerly winds tempered by the lake give the eastern shore a moderate climate, making it a rich fruit belt and popular resort area. Sand dunes border the eastern and southern shores of the lake; Indiana Dunes National Lakeshore (see National Parks and Monuments, table) is there. Ore, coal, and limestone are the main items moved on the lake. The Saint Lawrence Seaway has opened Lake Michigan to international trade. The southern part of the lake does not freeze over in the winter, but storms and ice halt interlake movement from December to April.
Lake Michigan was discovered in 1634 by the French explorer Jean Nicolet and was later explored by the French explorers Marquette and Jolliet. French missionary and trade centers thrived there by the late 1600s. As part of the bitterly contested Northwest Territory, the area passed to England in 1763 and later to the United States in 1796. The area was isolated until the 1830s, when improvements in transportation brought settlers there.