Great Lakes, a group of five large lakes in east-central North America. From the west, they are Lakes Superior, Michigan, Huron. Erie, and Ontario. Lake Michigan is entirely within the United States; the other four lakes, astride the international boundary, are shared by Canada and the United States.
The Great Lakes and the basin they drain occupy some 292,000 square miles (756,000 km 2 )—an area larger than that of Texas. The lakes themselves cover 94,250 square miles (244,100 km 2 ), making them collectively the world's largest body of freshwater. Lake Superior, with an area of 31,700 square miles (82,100 km 2 ), is the world's largest freshwater lake.
The five lakes descend in a series of steps, from Lake Superior's surface elevation of 600 feet (183 m) above sea level to Lake Ontario's 250 feet (76 m).
The Great Lakes were formed during the most recent Ice Age, during which much of North America was covered by a series of continental glaciers. As the last of the glaciers retreated northward some 10,000 to 15,000 years ago, the final shaping of the lakes took place. Former outlets from the lakes to the Mississippi and Hudson rivers were closed off, leaving only the St. Lawrence River to drain the entire basin.
The Great Lakes constitute one of the world's greatest inland waterway systems. They penetrate deeply into the rich farming and industrial regions of North America's heartland, providing convenient routes for the movement of vital cargoes such as iron ore, coal, and grain. With the opening of the St. Lawrence Seaway in 1959, the iron mines of Quebec and Labrador began to ship ore in large quantities to Great Lakes ports, supplementing the declining supplies of Minnesota iron ore. In turn, wheat from midcontinent plains and prairies goes directly to overseas ports from such places as Chicago and Duluth-Superior.
Shipping on the Great Lakes is virtually year-round, but there is a short period of closure in the winter during which maintenance work is done on ships and docks. The open waters in the lakes seldom freeze over.
Although their value as transport arteries is high, the Great Lakes serve numerous other purposes as well. They supply water for domestic and industrial uses, and their beaches, islands, and peninsulas provide hundreds of recreational areas.
Commercial fishing was much more common in the Great Lakes during the 19th century and the first half of the 20th century than it is today. Many species of commercially important fish disappeared or were reduced in number because of overfishing, pollution (from industrial wastes, agricultural pesticides, and other sources), and the spread of such nonnative species as the sea lamprey. ( )
In the early 1970's, the United States and Canada began programs to control the amount of pollution entering the lakes. Although pollution has been reduced and fish of some species have increased in number, many problems remain. The most serious problem is that some fish are contaminated with pollutants that may cause illness in persons who eat the fish.
Étienne Brûlé, a French explorer, was probably the first European to see the Great Lakes, when he explored what is now Lake Huron in 1612. In 1615 Samuel de Champlain journeyed up the Ottawa River and crossed overland to Georgian Bay on Lake Huron. Within a short time other Frenchmen were exploring the lakes region. A series of forts and a thriving fur trade were established. The term “great lakes” was first used by the French fur trader Pierre Esprit Radisson in 1665.
By the time the region's fur trade diminished in the mid-19th century, large deposits of copper and iron ore were found near the shores of Lake Superior. Difficulties in moving the ore around the rapids in the St. Marys River led to the construction of the first ship canal at Sault Ste. Marie, opened in 1855. This canal overcame the last major hindrance to shipping in the Great Lakes; the Welland Canal, bypassing Niagara Falls, had been opened in 1829.
After the mid-19th century, shipping on the lakes grew rapidly, and sailing ships began to give way to steam-powered lake freighters. At the same time some of the great industrial and commercial centers of North America were developing along or near the shores of the Great Lakes. Cities such as Chicago, Detroit, Cleveland, and Toronto grew primarily because their excellent water and rail transport facilities attracted manufacturing and other industries. In the 20th century the economic importance of the lakes continued to grow, and the opening of the St. Lawrence Seaway in 1959 brought increased foreign trade.