Great Lakes - Geography.

Great Lakes - Geography. I INTRODUCTION Great Lakes, group of five large freshwater lakes in central North America, interconnected by natural and artificial channels. From west to east they are Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario. Lake Michigan lies entirely within the United States; the others form part of the border between the United States and Canada. The combined surface area of the lakes is 244,100 sq km (94,250 sq mi). Together the lakes drain a total of about 750,000 sq km (about 290,000 sq mi) in Canada and the United States. The primary outlet of the system is the St. Lawrence River; a portion is diverted from Lake Michigan to the Chicago River. The lakes are bordered by the Canadian province of Ontario and by eight U.S. states: New York, Pennsylvania, Ohio, Indiana, Michigan, Illinois, Minnesota, and Wisconsin. The Great Lakes are a natural resource of tremendous significance in North America, serving as the focus of the industrial heartland of the continent. Together they hold about 20 percent of the world's fresh surface water. Four large cities in North America (Chicago, Detroit, Toronto, and Cleveland) lie on the shores of the Great Lakes system and owe much of their wealth to commerce attracted to the lakes. The lakes also form an important recreational resource with about 17,000 km (about 10,500 mi) of shoreline, rich sport fisheries, and numerous beaches and marinas. II DESCRIPTION Lake Superior, the largest in area of the Great Lakes at 82,100 sq km (31,700 sq mi), is the largest freshwater lake in the world. Of the Great Lakes, Lake Superior is the highest above sea level, at 183 m (600 ft), the farthest north, and the coldest. Its outlet is the Saint Marys River, which enters Lake Huron after falling about 7 m (about 21 ft) over a series of rapids between the twin cities of Sault Sainte Marie, in Ontario and Michigan. Lake elevations decrease to the south and east. Lake Huron and Lake Michigan lie at the same elevation, 176 m (577 ft), separated by the Straits of Mackinac, where water flows from Lake Michigan to Lake Huron. Huron is the larger of the two in area, at 59,600 sq km (23,000 sq mi); Michigan is deeper, 85 m (279 ft) on average, and contains more water. Both Michigan and Huron have numerous islands, the largest of which are contained in the Manitoulin Islands chain in Lake Huron. At its southern end, Lake Huron drains into the Saint Clair River, which falls about 3 m (about 9 ft) between Lake Huron and the small, shallow basin of Lake Saint Clair. The Detroit River connects Lake Saint Clair with Lake Erie. At its northeast end, Lake Erie empties into the Niagara River, which drops 99 m (325 ft) as it flows north to Lake Ontario. Lake Ontario, the smallest of the Great Lakes at 19,010 sq km (7340 sq mi), is the 14th largest lake in the world. The outlet of Lake Ontario is the St. Lawrence River. III SHIPPING AND OTHER ECONOMIC ACTIVITIES 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 the St. 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-sized oceangoing 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 is connected 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, the Des 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 these channels 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 steelproducing plants in the lower lakes region, especially the Chicago and Gary, Indiana, area; Detroit; Cleveland; Erie, Pennsylvania; and Hamilton, Ontario. However, iron production in Minnesota 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 lakes has 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 shipped from 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 fish populations in the lakes were severely depleted by the mid-1900s, and today there is little commercial fishing. The lakes are an important recreational resource. Thousands of 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 on Lake 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 shoreline erosion is a major problem. Low levels threaten shipping, power generation, recreational opportunities, and wildlife habitats. Lakes Superior and Ontario are the only lakes among 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 Sault Sainte 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 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. 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; RenéRobert 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. Contributed By: William H. Renwick Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.