Great Lakes

A

Pollution

Pollution in the lakes comes from many sources, including industrial discharges, municipal sewage, and agricultural runoff. During the 1960s increases in phosphorus in the lower lakes generated considerable public concern. The increases in phosphorus were caused both by agricultural use of fertilizers and by municipal wastewater discharges. Phosphorus contributes to the growth of algae in the lakes. This algae eventually decays and causes oxygen depletion in the water, which threatens certain species of fish. Meanwhile, other pollution-tolerant organisms thrive.

More recently, toxic contaminants, especially pesticides such as dichlorodiphenyl-trichloroethane (DDT) and industrial pollutants like polychlorinated biphenyls (PCBs), have drawn concern. In many areas residents are warned to limit their consumption of fish caught in the lakes because toxic substances tend to accumulate within marine life. Several agreements between the United States and Canada, including the Boundary Waters Treaty of 1909 and the Great Lakes Water Quality Agreements of 1972 and 1978, have focused on water-quality problems in the Great Lakes. The International Joint Commission on the Great Lakes, established under the Boundary Waters Treaty, implements and oversees these agreements and has limited authority to regulate obstructions or diversions of boundary waters that would affect the natural level or flow of lake waters.

B

Exotic Species

The fish populations of the lakes have changed dramatically in the 20th century; changes were wrought at first by overfishing and then by the introduction of exotic species. Most notable of the latter was the parasitic sea lamprey, which probably entered the lakes via the Erie Canal and spread following the completion of the new Welland Ship Canal in 1932. The sea lamprey virtually eliminated lake trout from Lakes Huron and Michigan. Canadian and American government programs, instituted in the 1970s, have reduced the number of lampreys.

The decline in the lake-trout population allowed another invader, the alewife, to flourish, unconstrained by any natural predators. Alewives entered the lakes through the St. Lawrence Seaway and the Welland Ship Canal. Alewife populations have been brought under control by the coho salmon, imported into the lakes in the 1970s, which has become the dominant predator and an important sport fish.

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In 1986, a small mollusk known as a zebra mussel was introduced from Europe, probably carried by a ship. The zebra mussel population grew rapidly. The mussels have coated pilings and clogged water intakes at power plants. Zebra mussels filter the water, consuming algae and potentially displacing other algae-feeding organisms. In removing algae from the water the mussels make the water much clearer. However, they also make the water more acidic and increase the risk of exposure for humans and wildlife to PCBs and other pollutants. As the mussels filter the lake water, they absorb the relatively low levels of toxic substances already in the water. Then when the mussels are eaten by fish or birds, the toxic substances move along the food chain.

V

History

The Great Lakes were formed by erosion and deposition during the repeated glacial advances and retreats of the Pleistocene Epoch (the most recent of the Ice Ages), which ended about 10,000 years ago. Before then, the area now occupied by Lake Superior was made of broad valleys and river systems, and the present region of the other lakes probably was a plain.

The lakes lie just southwest of the margins of the Canadian Shield, an area of resistant rocks that extends south to central Ontario and extreme northern Minnesota, Wisconsin, and Michigan. During the Ice Age, glaciers eroded the weaker rocks at the margins of the shield and deposited some of this material to the south of the lakes. These deposits dammed drainage that might have flowed to the south. Late in the Ice Age the glaciers prevented drainage to the northeast via the St. Lawrence River, and numerous overflow channels were formed as the water sought other routes to the sea. Eventually, some of these channels were exploited in the construction of canals, including the Illinois Waterway, the Erie Canal, the Champlain Canal connecting Lake Champlain with the Hudson River, and the Trent Canal system linking Lakes Ontario and Huron.

The Great Lakes region was home to numerous Native American groups who fished its waters and operated trade networks extending from present-day Minnesota to New York. These included the Ojibwa, Ottawa, Algonquin, Erie, Iroquois, Huron, Sac (Sauk), Fox, Winnebago, Kickapoo, and Potawatomi. The first Europeans to travel on the lakes were French missionaries and explorers between the mid-1500s and mid-1600s, such as Jacques Cartier; Étienne Brûlé; Samuel de Champlain; René-Robert Cavelier, Sieur de La Salle; and Jean Nicolet. Their travels opened up the fur trade, which exploited the lakes as a transport route to carry pelts by canoe from the interior to Atlantic ports. As early as the 1620s the French exerted control to the western margins of Lake Superior, establishing a fort at Detroit and tapping fur resources from Québec, in the east, to western Ontario and Minnesota, in the west.

During the War of 1812 (1812-1815), naval battles between the United States and Britain took place on Lakes Ontario and Erie. Since then lake-based commerce has developed peacefully. The lakes were an important route for westward expansion of European settlement during the early 1800s. The Welland Ship Canal, joining Lakes Erie and Ontario, was opened in 1829 as a means to bypass Niagara Falls, and the first significant canals at Sault Sainte Marie were built in the 1850s. By the late 1800s lake ports such as Chicago; Milwaukee, Wisconsin; Detroit; Cleveland; Buffalo, New York; Toronto; and Hamilton were thriving industrial cities linking the interior of the continent with the Atlantic seaboard. The St. Lawrence Seaway strengthened this link by permitting oceangoing vessels to travel between the lakes and the Atlantic.