United States:

Physical Geography

The conterminous United States may be divided into seven broad physiographic divisions: from east to west, the Atlantic–Gulf Coastal Plain; the Appalachian Highlands; the Interior Plains; the Interior Highlands; the Rocky Mountain System; the Intermontane Region; and the Pacific Mountain System. An eighth division, the Laurentian Uplands, a part of the Canadian Shield, dips into the United States from Canada in the Great Lakes region. It is an area of little local relief, with an irregular drainage system and many lakes, as well as some of the oldest exposed rocks in the United States.

The terrain of the N United States was formed by the great continental ice sheets that covered N North America during the late Cenozoic Era. The southern edge of the ice sheet is roughly traced by a line of terminal moraines extending west from E Long Island and then along the course of the Ohio and Missouri rivers to the Rocky Mts.; land north of this line is covered by glacial material. Alaska and the mountains of NW United States had extensive mountain glaciers and were heavily eroded. Large glacial lakes (see Lake Bonneville under Bonneville Salt Flats; Lahontan, Lake) occupied sections of the Basin and Range province; the Great Salt Lake and the other lakes of this region are remnants of the glacial lakes.

The Atlantic–Gulf Coastal Plain extends along the east and southeast coasts of the United States from E Long Island to the Rio Grande; Cape Cod and the islands off SE Massachusetts are also part of this region. Although narrow in the north, the Atlantic Coastal Plain widens in the south, merging with the Gulf Coastal Plain in Florida. The Atlantic and Gulf coasts are essentially coastlines of submergence, with numerous estuaries, embayments, islands, sandspits, and barrier beaches backed by lagoons. The northeast coast has many fine natural harbors, such as those of New York Bay and Chesapeake Bay, but south of the great capes of the North Carolina coast (Fear, Lookout, and Hatteras) there are few large bays. A principal feature of the lagoon-lined Gulf Coast is the great delta of the Mississippi River.

The Atlantic Coastal Plain rises in the west to the rolling Piedmont (the falls along which were an early source of waterpower), a hilly transitional zone leading to the Appalachian Mountains. These ancient mountains, a once towering system now worn low by erosion, extend southwest from SE Canada to the Gulf Coastal Plain in Alabama. In E New England, the Appalachians extend in a few places to the Atlantic Ocean, contributing to a rocky, irregular coastline. The Appalachians and the Adirondack Mountains of New York (which are geologically related to the Canadian Shield) include all the chief highlands of E United States; Mt. Mitchell (6,684 ft/2,037 m high), in the Black Mts. of North Carolina, is the highest point of E North America.

Extending more than 1,000 mi (1,610 km) from the Appalachians to the Rocky Mts. and lying between Canada (into which they extend) in the north and the Gulf Coastal Plain in the south are the undulating Interior Plains. Once covered by a great inland sea, the Interior Plains are underlain by sedimentary rock. Almost all of the region is drained by one of the world's greatest river systems—the Mississippi-Missouri. The Interior Plains may be divided into two sections: the fertile central lowlands, the agricultural heartland of the United States; and the Great Plains, a treeless plateau that gently rises from the central lowlands to the foothills of the Rocky Mts. The Black Hills of South Dakota form the region's only upland area.

The Interior Highlands are located just W of the Mississippi River between the Interior Plains and the Gulf Coastal Plain. This region consists of the rolling Ozark Plateau (see Ozarks) to the north and the Ouachita Mountains, which are similar in structure to the ridge and valley section of the Appalachians, to the east.

West of the Great Plains are the lofty Rocky Mountains. This geologically young and complex system extends into NW United States from Canada and runs S into New Mexico. There are numerous high peaks in the Rockies; the highest is Mt. Elbert (14,433 ft/4,399 m). The Rocky Mts. are divided into four sections—the Northern Rockies, the Middle Rockies, the Wyoming (Great Divide) Basin, and the Southern Rockies. Along the crest of the Rockies is the Continental Divide, separating Atlantic-bound drainage from that heading for the Pacific Ocean.

Between the Rocky Mts. and the ranges to the west is the Intermontane Region, an arid expanse of plateaus, basins, and ranges. The Columbia Plateau, in the north of the region, was formed by volcanic lava and is drained by the Columbia River and its tributary the Snake River, both of which have cut deep canyons into the plateau. The enormous Colorado Plateau, an area of sedimentary rock, is drained by the Colorado River and its tributaries; there the Colorado River has entrenched itself to form the Grand Canyon, one of the world's most impressive scenic wonders. West of the plateaus is the Basin and Range province, an area of extensive semidesert.

The lowest point in North America, in Death Valley (282 ft/86 m below sea level), is there. The largest basin in the region is the Great Basin, an area of interior drainage (the Humboldt River is the largest stream) and of numerous salt lakes, including the Great Salt Lake. Between the Intermontane Region and the Pacific Ocean is the Pacific Mountain System, a series of ranges generally paralleling the coast, formed by faulting and volcanism. The Cascade Range, with its numerous volcanic peaks extends S from SW Canada into N California, and from there is continued south by the Sierra Nevada, a great fault block. Mt. Whitney (14,495 ft/4,418 m), in the Sierra Nevada, is the highest peak in the conterminous United States.

West of the Cascades and the Sierra Nevada and separated from them by a structural trough are the Coast Ranges, which extend along the length of the U.S. Pacific coast. The Central Valley in California, the Willamette Valley in Oregon, and the Puget Sound lowlands in Washington are part of the trough. The San Andreas Fault, a fracture in the earth's crust, parallels the trend of the Coast Ranges from San Francisco Bay SE to NW Mexico; earthquakes are common along its entire length. The Pacific Coastal Plain is narrow, and in many cases the mountains plunge directly into the sea. A coastline of emergence, it has few islands, except for California's Channel Islands and those in Puget Sound; there are few good harbors besides Puget Sound, San Francisco Bay, and San Diego Bay.

Alaska may be divided into four physiographic regions; they are, from north to south, the Arctic Lowlands, the coastal plain of the Arctic Ocean; the Rocky Mountain System, of which the Brooks Range is the northernmost section; the Central Basins and Highlands Region, which is dominated by the Yukon River basin; and the Pacific Mountain System, which parallels Alaska's southern coast and which rises to 20,310 ft (6,190 m) at Denali (Mt. McKinley), the highest peak of North America. The islands of SE Alaska and those of the Aleutian Islands chain are partially submerged portions of the Pacific Mountain System and are frequently subjected to volcanic activity and earthquakes. These islands, like those of Hawaii, are the tops of volcanoes that rise from the floor of the Pacific Ocean. Mauna Kea and Mauna Loa on Hawaii are active volcanoes; the other Hawaiian islands are extinct volcanoes.

The United States has an extensive inland waterway system, much of which has been improved for navigation and flood control and developed to produce hydroelectricity and irrigation water by such agencies as the U.S. Bureau of Reclamation, the U.S. Army Corps of Engineers, and the Tennessee Valley Authority. Some of the world's larger dams, artificial lakes, and hydroelectric power plants are on U.S. rivers. The Mississippi-Missouri river system (c.3,890 mi/6,300 km long), is the longest in the United States and the second longest in the world. With its hundreds of tributaries, chief among which are the Red River, the Ohio, and the Arkansas, the Mississippi basin drains more than half of the nation. The Yukon, Columbia, Colorado, and Rio Grande also have huge drainage basins. Other notable river systems include the Connecticut, Hudson, Delaware, Susquehanna, Potomac, James, Alabama, Trinity, San Joaquin, and Sacramento.

The Great Salt Lake and Alaska's Iliamna are the largest U.S. lakes outside the Great Lakes and Lake of the Woods, which are shared with Canada (Lake Michigan and Iliamna are the largest freshwater lakes entirely within the United States). The Illinois Waterway connects the Great Lakes with the Mississippi River, and the New York State Canal System links them with the Hudson. The Intracoastal Waterway provides sheltered passage for shallow draft vessels along the Atlantic and Gulf Coasts.

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