Great Lakes
The advance and retreat of glaciers over millions of years scraped and scoured the Great Lakes basins until they attained their present form about 10,000 years ago. Forming the largest system of inland lakes in the world, the Great Lakes have a surface area of 94,200 mi2 (244,000 km2) and a volume of more than 28 trillion yd4 (22,000 km3) of water, 20 percent of the world's surface freshwater.
Lake Superior, with more than 31,660 mi2 (82,000 km2) of water, has the largest surface area of freshwater on earth. Lake Huron, the world's fifth largest lake, is at the same elevation and about the same size as Lake Michigan, the world's sixth largest lake. The two are joined by the narrow, deep Straits of Mackinac. Their accumulated waters empty into the St. Clair River which flows into the 460 mi2(1,190 km2) Lake St. Clair. The water continues its flow into the Detroit River before entering Lake Erie, the eleventh largest lake in the world. It is the oldest, shallowest, busiest, and most eutrophic of the Great Lakes. The waterway continues on into the Niagara River, then to the famous Niagara Falls, where the water descends a total of 325 ft (99 m) before it empties into the last Great Lake, Ontario. The fourteenth largest lake on earth, Lake Ontario is the smallest in surface area but the second deepest of the Great Lakes. It discharges into the St. Lawrence River, which flows into the Atlantic Ocean at the Gulf of St. Lawrence.
The first European explorers discovered a great variety of native fish. Approximately 153 species were eventually identified before human interference disrupted the ecosystem, first by overfishing, and then by lumbering and industrial development. As many species of fish have disappeared, about twenty new species have been introduced. Some, such as the Pacific salmonids, carp, and smelt, were introduced intentionally. Others, such as the sea lamprey, alewife, and zebra mussel, gained access through the Erie and Welland Canals or by release with the ballast water of vessels transporting other cargo.
Today, lake trout, burbot, and whitefish are the principal catches of a once extraordinarily rich fisheries enterprise. Despite the decline in the quality and numbers of suitable fish, sport and commercial fishing are still vital Great Lakes industries. The sport fishery consists primarily of coho, chinook salmon, steelhead trout, walleye, and perch. They now attract about five million anglers annually with a regional economic benefit of about $2 billion.
Besides directly water-related activities, presently, one-fifth of the industry and commerce of the United States is located in the Great Lakes catchment basin because of the availability of abundant cheap and clean freshwater and accessible, efficient water transportation among the lakes and to the oceans. As a consequence, pollution has taken some obvious as well as more subtle forms. Using the lakes as a cheap sewage disposal site for shoreline city populations began in the early seventeenth and eighteenth centuries and continued until the early 1970s. To improve the quality of the Great Lakes, the first efforts concentrated on preventing or removing conventional pollutants such as phosphates, suspended solids, and nitrogen.
More deadly toxic contaminants often are not visible and so initially attracted less attention. Over the past fifty years municipal and industrial wastes so polluted the waters, especially the lower Great Lakes, that, beginning in the middle 1960s organochlorides were identified as serious contaminants. Fish were collecting, through bioaccumulation, relatively large concentrations of agricultural pesticides such as DDT and dieldrin as well as the industrial chemical polychlorinated biphenyl (PCB) in their tissues. These were passed into the human food chain/web. By 1980 more than 400 organic and heavy metals contaminants had been found in fish, and fishermen were warned to limit their consumption. The effects of pollutants are seen primarily at the tops of food chains and are usually discovered through changes in population levels of predator species. Organochlorines and methylated mercury, for example, bioaccumulate to levels that may cause reproductive failures in fish-eating birds and animals such as cormorants, eagles, and mink.
Between 1969 and 1972 legislation was enacted in several states bordering the Great Lakes basin to restrict or ban the use of dieldrin, DDT, PCBs, mercury and other toxic chemicals. After point source discharges were regulated, lake trout and chub, especially in Lake Michigan, showed dramatic declines in these contaminants. By 1978–79, however, the fish contaminant declines were only slight; or the levels remained relatively constant, reflecting airborne inputs as well as the remobilization of contaminants from the sediment.
This problem is likely to continue because the turnover rates of the Great Lakes are very slow; and mercury, PCBs and the pesticides DDT, dieldrin and chlordane are very resistant to degradation in the environment. Also, these compounds continue to enter the Great Lakes ecosystem from highly diffuse nonpoint sources such as airborne deposition, agricultural and urban runoff, remobilization from the sediments, leaching from municipal and industrial landfills, municipal and industrial discharges, and illegal dumping.
Agricultural Chemicals; Agricultural Pollution; Great Lakes Water Quality Agreement; Heavy Metals and Heavy Metal Poisoning; Industrial Waste Treatment; Methylation; Water Pollution
Resources
Books
Ashworth, W. The Late, Great Lakes: An Environmental History. New York: Knopf, 1986.
Great Lakes, Great Legacy. Washington, DC: Conservation Foundation, 1989.
Hough, J. L. Geology of the Great Lakes. Urbana, IL: University of Illinois Press, 1958.
Sixth Biennial Report on Great Lakes Water Quality. Windsor, Ont.: International Joint Commission, 1992.
Weller, P. Fresh Water Seas: Saving the Great Lakes, Between the Lines. Toronto: Publishers, 1990.
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