Great Lakes - geography.

« Boats on Lake MichiganLake Michigan is the only one of the five Great Lakes that is entirely within the borders of the United States.

It is used for commercialshipping as well as recreation.

This view of the lake includes the city of Chicago in the background.Illinois Department of Commerce and Community Affairs 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 theSt.

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-sizedoceangoing 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 isconnected 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, theDes 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 thesechannels 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 inthe lower lakes region, especially the Chicago and Gary, Indiana, area; Detroit; Cleveland; Erie, Pennsylvania; and Hamilton, Ontario.

However, iron production inMinnesota 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 lakeshas 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 shippedfrom 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 fishpopulations in the lakes were severely depleted by the mid-1900s, and today there is little commercial fishing.

The lakes are an important recreational resource.

Thousandsof 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 onLake 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 shorelineerosion is a major problem.

Low levels threaten shipping, power generation, recreational opportunities, and wildlife habitats.

Lakes Superior and Ontario are the only lakesamong 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 SaultSainte Marie, while Lake Ontario is regulated by a dam at Kingston, Ontario. 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 thelower 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), havedrawn 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 and1978, 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 oflake 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 ShipCanal in 1932.

The sea lamprey virtually eliminated lake trout from Lakes Huron and Michigan.

Canadian and American government programs, instituted in the 1970s, havereduced 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 hasbecome the dominant predator and an important sport fish. 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 havecoated pilings and clogged water intakes at power plants.

Zebra mussels filter the water, consuming algae and potentially displacing other algae-feeding organisms.

Inremoving 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 andwildlife 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. »