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The Natural Environment

Drainage

The Continental, or Great, Divide, which mainly runs along the crest of the Rocky Mountains, splits North America into two great drainage basins. To the east of the divide, water flows toward the Arctic Ocean, Hudson Bay, the Atlantic Ocean, and the Gulf of Mexico, and to the west, rivers flow toward the Pacific Ocean.

Two prominent drainage systemsóthe Great Lakes-St. Lawrence River system and the Mississippi-Missouri river systemódominate the hydrography of eastern and central North America. The five Great Lakes (Superior, Michigan, Huron, Erie, and Ontario) drain northeast to the Atlantic Ocean via the relatively short St. Lawrence River. Most of the central part of the United States and a small part of southern Canada are drained south to the Gulf of Mexico by the Mississippi River and its tributaries, notably the Missouri River. A great many short, but often voluminous, rivers flow to the Atlantic and Gulf of Mexico along the well-watered eastern coasts of Canada, the United States, and Mexico. The northern interior of the continent is drained by the great Mackenzie River system of western Canada and by the numerous rivers that flow into Hudson Bay. To the west of the Continental Divide are relatively few major rivers (notably the Colorado, Columbia, Fraser, and Yukon) and many short, large-volume streams.

The southern half of North America contains only a few large natural lakes, but Canada and the northern United States have a vast number of sizable lakes. Lake Superior, the world's biggest freshwater lake, and 10 of the next 25 largest natural lakes are found in this region. Lake Mead, on the Colorado River in the United States, is a large artificial lake, and Great Salt Lake, in Utah, is noted for its highly saline water.

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