Great Lakes

I

Introduction

Great Lakes, group of five large lakes, central North America. From east to west, they are Lakes Ontario, Erie, Huron, Michigan, and Superior. Lake Michigan lies entirely within the United States, but the other lakes straddle the border between the United States and Canada. All are jointly controlled by the United States and Canada under the Boundary Waters Treaty of 1909. The total surface area of the Great Lakes is 244,108 sq km (94,251 sq mi), making them collectively the largest body of fresh water in the world. Together they hold about 20 per cent of the world’s fresh surface water; approximately two thirds of their surface area is within the boundaries of the United States. The lakes drain a basin of about 751,100 sq km (290,000 sq mi), an area slightly less than the size of Turkey. They serve as the focus for the industrial heartland of North America, and also provide an important recreational resource with about 17,000 km (about 10,625 mi) of shoreline. For further details see the separate article on each lake.

II

Lake Shipping

The Great Lakes, interconnected by rivers, straits, and canals, together form one of the world's busiest shipping arteries. They are linked with the Atlantic Ocean by the St Lawrence River, and, since the completion in 1959 of the St Lawrence Seaway, they have been open to medium-sized ocean-going vessels, allowing them to travel as far west as Duluth, Minnesota. In addition, Lake Erie is connected with the Atlantic by way of the Erie Canal and the Hudson River, and Lake Michigan is linked with the Gulf of Mexico via the Illinois Waterway and the Mississippi River. Interlake waterways include the Welland Ship Canal, which bypasses Niagara Falls to link Lakes Ontario and Erie, and the Sault Sainte Marie Canals, which permit navigation between Lakes Huron and Superior.

Specially designed long, narrow vessels, called lakers, handle most of the freight carried on the lakes, which are closed to shipping by ice during the winter. The principal cargoes transported are iron ore and grain. Although iron and steel production have decreased in recent years, iron ore is still the largest single cargo shipped on the lakes; it is transported from the Lake Superior region to ports on the southern shore of Lake Erie, from which it is sent to the steel mills of Ohio, Pennsylvania, and Indiana. Vast quantities of grain produced in the upper Midwest of the United States and in the Prairie provinces of Canada are transported via the lakes, principally from Thunder Bay, Ontario, to processing centres such as Buffalo, New York, on Lake Erie, and also abroad. Coal, limestone, and manufactured goods are also shipped on the lakes. Most of the cargo is carried from west to east. About 10-20 per cent of the freight shipped from Great Lakes’ ports passes through the St Lawrence Seaway.

III

Pollution

Pollution in the lakes comes from many sources, including industrial waste, municipal sewage, and agricultural run-off. During the 1960s increases in phosphorus in the lower lakes generated considerable public concern. These increases were caused both by agricultural use of fertilizers and by municipal wastewater discharges. Phosphorus contributes to the growth of algae in the lakes. These algae eventually decay and cause 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 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. In 2001 there were indications that some pesticides and PCBs were being released from the lakes in a process of self-cleaning called outgassing; as the chemicals do not break down easily, however, further concerns arose about potential atmospheric pollution in the area.

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.

IV

Fishing and Exotic Species

Although in the past the lakes have supported important commercial fisheries, the fish populations of the lakes changed dramatically in the 20th century and today there is little commercial fishing. Changes were wrought 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 Welland Ship Canal in 1932. The sea lamprey virtually eliminated lake trout from Lakes Huron and Michigan. Canadian and American government programmes, 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 game fish.

In 1986, a small mollusc 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 stations. 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, which can then be passed along the food chain.