a group of large lakes in the eastern part of North America, in the St. Lawrence River basin. They consist of Lakes Superior, Huron, Michigan, Erie, and Ontario. Lake Michigan is entirely within the USA, and the border between the USA and Canada runs through the rest of the lakes and the short rivers that connect them. About one-third of the water area belongs to Canada.
| Table 1. Great Lakes | |||
|---|---|---|---|
| Altitude above sea level (m) | Area (thousand sq km) | Greatest depth (m) | |
| Superior ............... | 183 | 82.4 | 393 |
| Huron ............... | 177 | 59.6 | 208 |
| Michigan ............... | 177 | 58.0 | 281 |
| Erie ............... | 174 | 25.7 | 64 |
| Ontario ............... | 75 | 19.5 | 236 |
The shore line (with islands) is about 18,000 km long. The Great Lakes are the largest accumulation of fresh water on earth. Their area is 245,200 sq km, and the area of their basin (including the lakes themselves) is 768,000 sq km. Their volume of water is 22,725 cubic km. Four lakes have depths greater than 200 m, and only in Lake Erie is the greatest depth 64 m. The largest and deepest of the lakes is Lake Superior, and the smallest is Lake Ontario. The Great Lakes are situated on different levels, but the height difference of the first four (see Table 1) does not exceed 9 m in all (183 m, Lake Superior and 174 m, Lake Erie), and only the lowest lake, Ontario, is situated almost 100 m below Lake Erie. The lakes are united by short, large-volume rivers with many rapids; the Ste. Marie River unites Lake Superior and Lake Huron (112 km long); the St. Clair River (43 km), Lake St. Clair (1,275 sq km in area), and the Detroit River (51 km) unite Lake Huron and Lake Erie; and the Niagara River (54 km), which forms Niagara Falls (c. 50 m high), unites Lake Erie and Lake Ontario. Lakes Huron and Michigan, which are at the same altitude, are joined by the Straits of Mackinac, which are about 3 km wide. Several hundred small rivers fall into the Great Lakes, which drain through the St. Lawrence River and flow from Lake Ontario into the Atlantic Ocean. The average outflow of water at its source is 6,637 cu m per sec.
The basins of Lake Superior and the northern part of Lake Huron were formed from the crystalline rocks of the southern part of the Canadian shield, whereas those of the remaining lakes were formed from a mass of limestones, dolomite, and sandstones of the Paleozoic North American platform. These basins formed as a result of tectonic movement and pre-Ice Age and Ice Age river and glacial erosion. The origin of the Great Lakes water mass is related to the melting of the glacial cover, the recession of which formed in the area a series of large lakes, which changed their outlines many times. The northern part of the shore line is broken, and the islands and shores (to 400 m high) are rocky, precipitous, and very picturesque. This is particularly true of the shores of Lake Superior and the northern parts of Lake Huron. The shores of the southern and southeastern parts of the lakes are primarily low, clayey, and sandy.
Fluctuations of the level of the Great Lakes is artificially regulated for purposes of navigation, power generation, and so on. The amplitude of seasonal fluctuation is 30-60 cm; the highest levels come in the summer and the lowest in winter. Short-term fluctuations in the water level caused by strong winds and seiches reach 3-4 m. The height of the tides is 3-4 cm (Lakes Superior and Michigan). The lakes freeze only in the shore zone (from December to March or April). As a result of frequent fall and winter storms the central parts of the lakes do not remain completely frozen over. The duration of navigability is eight to nine months a year.
In the lakes are found 173 species of fish, mainly of the carp, perch, and salmon families. Whitefish and trout are of economic significance.
In the south and southeast heavily settled industrial regions of the USA and Canada border on the Great Lakes; there are agricultural and raw-material regions in the north and west. Some of the largest cities of the USA are situated on the Great Lakes; Chicago and Milwaukee are on Lake Michigan; Buffalo and Cleveland are on Lake Erie; the center of the auto industry, Detroit, is on the river of the same name near its entrance into Lake Erie; and the second largest city in Canada, Toronto, is on Lake Ontario. The Great Lakes are joined to the Mississippi River basin by a system of navigable canals that begin near Chicago on Lake Michigan and to the Hudson River (on which New York is located) by a canal beginning in Lake Erie near Buffalo.
The Great Lakes form a deepwater internal shipping route 1,873 km long (from the largest port, Duluth, on Lake Superior to the source of the St. Lawrence River), by means of locks that bypass rapids on the Ste. Marie River (the Sault Ste. Marie Canal) and Niagara Falls on the Niagara river (Welland canal).
As a result of the completion in 1959 of the reconstruction of the canals built to avoid the rapids of the St. Lawrence River, a continuous water route from the Great Lakes to the Atlantic Ocean was created. Its length is about 3,000 km, and its depth is not less than 8 m, thus making it available to large oceangoing vessels. The main flow of cargo is from west to east (wheat and iron ore); the primary freight moving from east to west is coal. The annual freight turnover on the Great Lakes is about 200 million tons, of which more than half takes place in Lake Superior. The freight is carried mainly in ships belonging to American companies. The tonnage of the Canadian Merchant Marine on the Great Lakes constitutes only one-fourth of the total lake fleet tonnage.
O. A. SPENGLER