Great Lakes

I

Introduction

Great Lakes, group of five large freshwater lakes in central North America, interconnected by natural and artificial channels. From west to east they are Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario. Lake Michigan lies entirely within the United States; the others form part of the border between the United States and Canada. The combined surface area of the lakes is 244,100 sq km (94,250 sq mi). Together the lakes drain a total of about 750,000 sq km (about 290,000 sq mi) in Canada and the United States. The primary outlet of the system is the St. Lawrence River; a portion is diverted from Lake Michigan to the Chicago River. The lakes are bordered by the Canadian province of Ontario and by eight U.S. states: New York, Pennsylvania, Ohio, Indiana, Michigan, Illinois, Minnesota, and Wisconsin.

The Great Lakes are a natural resource of tremendous significance in North America, serving as the focus of the industrial heartland of the continent. Together they hold about 20 percent of the world’s fresh surface water. Four large cities in North America (Chicago, Detroit, Toronto, and Cleveland) lie on the shores of the Great Lakes system and owe much of their wealth to commerce attracted to the lakes. The lakes also form an important recreational resource with about 17,000 km (about 10,500 mi) of shoreline, rich sport fisheries, and numerous beaches and marinas.

II

Description

Lake Superior, the largest in area of the Great Lakes at 82,100 sq km (31,700 sq mi), is the largest freshwater lake in the world. Of the Great Lakes, Lake Superior is the highest above sea level, at 183 m (600 ft), the farthest north, and the coldest. Its outlet is the Saint Marys River, which enters Lake Huron after falling about 7 m (about 21 ft) over a series of rapids between the twin cities of Sault Sainte Marie, in Ontario and Michigan.

Lake elevations decrease to the south and east. Lake Huron and Lake Michigan lie at the same elevation, 176 m (577 ft), separated by the Straits of Mackinac, where water flows from Lake Michigan to Lake Huron. Huron is the larger of the two in area, at 59,600 sq km (23,000 sq mi); Michigan is deeper, 85 m (279 ft) on average, and contains more water. Both Michigan and Huron have numerous islands, the largest of which are contained in the Manitoulin Islands chain in Lake Huron. At its southern end, Lake Huron drains into the Saint Clair River, which falls about 3 m (about 9 ft) between Lake Huron and the small, shallow basin of Lake Saint Clair.

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The Detroit River connects Lake Saint Clair with Lake Erie. At its northeast end, Lake Erie empties into the Niagara River, which drops 99 m (325 ft) as it flows north to Lake Ontario. Lake Ontario, the smallest of the Great Lakes at 19,010 sq km (7340 sq mi), is the 14th largest lake in the world. The outlet of Lake Ontario is the St. Lawrence River.

III

Shipping and Other Economic Activities

The Great Lakes, interconnected by rivers, straits, and canals, together form one of the world’s busiest shipping arteries. The lakes are linked with the Atlantic Ocean via the St. Lawrence River. Since the completion in 1959 of the St. Lawrence Seaway, a system of dredged channels, canals, and locks, the lakes have been open to medium-sized oceangoing vessels.

Several other important channels facilitate commerce on the lakes. Lake Erie is connected with the Atlantic by way of the Erie Canal and the Hudson River. Lake Michigan is connected with the Mississippi River and the Gulf of Mexico by way of the Illinois Waterway, which encompasses the Chicago River, the Chicago Sanitary and Ship Canal, the Des Plaines River, and the Illinois River. The Sault Sainte Marie Canals allow ships to pass around the rapids in the Saint Marys River between Lakes Superior and Huron, while the Welland Ship Canal connects Lakes Erie and Ontario, bypassing Niagara Falls. Between 50 million and 100 million metric tons of freight pass through these channels each year; the lakes and channels are closed to shipping between December and April, when ice could impede passage.

Specially designed long narrow vessels carry most of the freight on the lakes. Historically, the Great Lakes have been a major route for shipment of iron ore from Minnesota, northwest Ontario, and Labrador (an area including northern Québec and the mainland portion of the province of Newfoundland and Labrador) to steel-producing plants in the lower lakes region, especially the Chicago and Gary, Indiana, area; Detroit; Cleveland; Erie, Pennsylvania; and Hamilton, Ontario. However, iron production in Minnesota has declined in recent years, as has steel production in areas bordering the southern portions of Lakes Michigan and Erie. Therefore, ore transport on the lakes has declined significantly. However, it is still the largest single cargo shipped on the lakes.

Grain grown in the Great Plains is another important cargo. It is shipped principally from Duluth, Minnesota, to ports on the lower lakes and to foreign markets via the St. Lawrence Seaway. Coal, limestone, petroleum products, and general cargo make up most of the rest of the cargo on the lakes. About 10 to 20 percent of the freight shipped from Great Lakes’ ports passes through the seaway to the Atlantic.

In the past the Great Lakes supported important commercial fisheries, with plentiful lake trout, sturgeon, whitefish, lake herring, pike, and walleye. Most of the native fish populations in the lakes were severely depleted by the mid-1900s, and today there is little commercial fishing. The lakes are an important recreational resource. Thousands of summer and year-round homes line the shores of the lakes, and in summer millions of people flock to the lakes for powerboating, sailing, fishing, and swimming.

IV

Problems

Water levels on the lakes vary over periods of several years by as much as 1 m (3 ft), and during storms lake levels may rise or fall as much as 2 m (7 ft), especially on Lake Erie, the shallowest of the Great Lakes. Long-term variations in lake levels are caused primarily by variations in precipitation. At times of high lake levels shoreline erosion is a major problem. Low levels threaten shipping, power generation, recreational opportunities, and wildlife habitats. Lakes Superior and Ontario are the only lakes among the Great Lakes whose water levels are regulated for hydroelectric-power generation. Lake Superior’s levels are controlled by gates on the Saint Marys River at Sault Sainte Marie, while Lake Ontario is regulated by a dam at Kingston, Ontario.