Great Lakes Fact Sheet
The Great Lakes contain about 23,000 km3 (5,500 cu.mi.) of water, covering a total area of 244,000 km2 (94,000 sq. mi.) The Great Lakes are the largest system of fresh, surface water on Earth, containing roughly 21 percent of the world supply and 84 percent of North America's supply. Only the polar ice caps contain more fresh water.
The Great Lakes basin is home to 25 million people in the United States and 8.5 million in Canada. This means that about 10 percent of the U.S. population and 31 percent of the Canadian population live in the Great Lakes basin.
In spite of their large size, the Great Lakes are sensitive to the effects of a wide range of pollutants. The sources of pollution include the runoff of soils and farm chemicals from agricultural lands, the waste from cities, discharges from industrial areas and leachate from disposal sites. The large surface area of the lakes also makes them vulnerable to direct atmospheric pollutants that fall as rain, snow, or dust on the lake surface, or exchange as gases with the lake water.
Some Physical Features of the Great Lakes
|Land Drainage Areab||miles2||49,300||45,600||51,700||30,140||24,720||201,460|
Outflows from the Great Lakes are relatively small (less than 1 percent per year) in comparison with the total volume of water. Pollutants that enter the lakes - whether by direct discharge along the shores, through tributaries, from land use or from the atmosphere - are retained in the system and become more concentrated with time. Also, pollutants remain in the system because of resuspension (or mixing back into the water) of sediment and cycling through biological food chains.
Lake Superior is the largest in terms of volume. It is also the deepest and coldest of the five. Because of its size, Superior has a retention time of 191 years. Retention time is a measure based on the volume of water in the lake and the mean rate of outflow. Most of the Superior basin is forested, with little agriculture because of a cool climate and poor soils. The forests and sparse population result in relatively few pollutants entering Lake Superior, except through airborne transport [ Lake Superior bathymetric map].
Lake Michigan, the second largest, is the only Great Lake entirely within the United States. The northern part is in the colder, less developed upper Great Lakes region. It is sparsely populated, except for the Fox River Valley, which drains into Green Bay. This bay has one of the most productive Great Lakes fisheries but receives the wastes from the world's largest concentration of pulp and paper mills. The more temperate southern basin of Lake Michigan is among the most urbanized areas in the Great Lakes system. It contains the Milwaukee and Chicago metropolitan areas. This region is home to about 8 million people or about one-fifth of the total population of the Great Lakes basin [ Lake Michigan bathymetric map].
Lake Huron, which includes Georgian Bay, is the third largest of the lakes by volume. Many Canadians and Americans own cottages on the shallow, sandy beaches of Huron and along the rocky shores of Georgian Bay. The Saginaw River basin is intensively farmed and contains the Flint and Saginaw-Bay City metropolitan areas. Saginaw Bay, like Green Bay, contains a very productive fishery [ Lake Huron bathymetric map].
Lake Erie is the smallest of the lakes in volume and is exposed to the greatest effects from urbanization and agriculture. Because of the fertile soils surrounding the lake, the area is intensively farmed. The lake receives runoff from the agricultural area of southwestern Ontario and parts of Ohio, Indiana and Michigan. Seventeen metropolitan areas with populations over 50,000 are located within the Lake Erie basin. Although the area of the lake is about 26,000 km2 (10,000 square miles), the average depth is only about 19 meters (62 feet). It is the shallowest of the five lakes and therefore warms rapidly in the spring and summer, and frequently freezes over in winter. It also has the shortest retention time of the lakes, 2.6 years. The western basin, comprising about one-fifth of the lake, is very shallow with an average depth of 7.4 meters (24 feet) and a maximum depth of 19 meters (62 feet) [ Lake Erie bathymetric map].
Lake Ontario, although slightly smaller in area, is much deeper than its upstream neighbor, Lake Erie, with an average depth of 86 meters (283 feet) and a retention time of about 6 years. Major urban industrial centers, such as Hamilton and Toronto, are located on its shore. The U.S. shore is less urbanized and is not intensively farmed, except for a narrow band along the lake [ Lake Ontario bathymetric map].
See the Great Lakes Atlas for much more information on the Great Lakes, their setting, people, environment, and management.