Lake Michigan is one of the five Great Lakes of North America and the only one located entirely within the United States. The other four Great Lakes are shared by the U.S. and Canada. It is the second-largest of the Great Lakes by volume and the third-largest by surface area, after Lake Superior and Lake Huron (and is slightly smaller than the U.S. state of West Virginia). To the east, its basin is conjoined with that of Lake Huron through the wide Straits of Mackinac, giving it the same surface elevation as its easterly counterpart; the two are technically a single lake. Lake Michigan is shared, from west to east, by the U.S. states of Wisconsin, Illinois,Indiana, and Michigan. Ports along its shores include Chicago; Milwaukee;Green Bay, Wisconsin; Gary, Indiana; and Benton Harbor, Michigan. The word "Michigan" originally referred to the lake itself, and is believed to come from the Ojibwa word "mishigami" meaning "great water".

The Great Lakes are also connected by the Illinois Waterway to the Gulf of Mexico via the Illinois River (from Chicago) and the Mississippi River. An alternate track is via the Illinois River (from Chicago), to the Mississippi, up the Ohio, and then through the Tennessee-Tombigbee Waterway (combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf. Commercial tug-and-barge traffic on these waterways is heavy.

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