Wisconsin Sea Grant: Gifts of the Glaciers

Thousands of years ago, the melting mile-thick glaciers of the Wisconsin Ice Age left the North American continent a magnificent gift: five huge freshwater seas collectively known today as the Great Lakes —- Lake Superior, Lake Huron, Lake Michigan, Lake Erie and Lake Ontario.

From the westernmost tip of Lake Superior at Duluth, Minnesota, to the easternmost tip of Lake Ontario at Watertown, New York, the five lakes stretch a thousand miles across the heartland of both the United States and Canada, creating nearly 9,500 miles of ocean-like shores. The lakes also contain an estimated 35,000 islands.

Dubbed "the nation's fourth seacoast," the U.S. Great Lakes shoreline alone totals more than 4,500 miles — longer than the U.S. East and Gulf coasts combined. As seen from space, they constitute one of the most identifiable features of the North American continent as well as the planet Earth.

The North American Great Lakes are unique among the world's large lakes in that their basins are linked together and form one continuous drainage basin. Starting in Lake Superior, the water flows out the lake's southeastern tip down the St. Marys River into Lakes Michigan and Huron, which actually are two halves of one lake. From there, the water flows southward down the St. Clair River at the southern tip of Lake Huron through "little" Lake St. Clair and the Detroit River to Lake Erie. Leaving Lake Erie, it flows north via the Niagara River and over Niagara Falls into Lake Ontario. It then flows northeast down the St. Lawrence River — the last link in a 2,000-mile-long waterway that ultimately connects Minnesota to the Atlantic Ocean.

However, this stair-step arrangement of basins is relatively new and resulted from the slow rise of the land as it rebounded from the depressing weight of the mile-thick ice sheets.

Lake Erie and southern Lake Michigan (Lake Chicago) were first unveiled by the glacier about 10,000 years ago. Both originally drained to the southwest, out the Maumee-Wabash-Ohio and Des Plaines-Illinois Rivers, respectively, to the Mississippi River. About 9,000 years ago, the early stage of Lake Superior, called Lake Duluth, drained southwest out the St. Croix and Mississippi Rivers, along what is today the Minnesota-Wisconsin border.

About 7,000 years ago, as the last ice left Lake Michigan, the land south of the lakes had risen high enough that the lakes no longer drained in that direction. Lake Ontario came into being, and the Niagara River became Lake Erie's outlet.

continued--As the glacier retreated into Canada...

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