Although the northern New England coast is rocky, along the rest of the eastern seaboard the Atlantic Coastal Plain rises gradually from the shoreline. Narrow in the north, the plain widens to about 320 km (200 mi) in the south and in Georgia merges with the Gulf Coastal Plain that borders the Gulf of Mexico and extends through Mexico as far as the Yucatán. West of the Atlantic Coastal Plain is the Piedmont Plateau, bounded by the Appalachian Mountains. The Appalachians, which extend from southwest Maine into central Alabama—with special names in some areas—are old mountains, largely eroded away, with rounded contours and forested, as a rule, to the top. Few of their summits rise much above 1,100 m (3,500 ft), although the highest, Mt. Mitchell in North Carolina, reaches 2,037 m (6,684 ft).
Between the Appalachians and the Rocky Mountains, more than 1,600 km (1,000 mi) to the west, lies the vast interior plain of the United States. Running south through the center of this plain and draining almost two-thirds of the area of the continental United States is the Mississippi River. Waters starting from the source of the Missouri, the longest of its tributaries, travel almost 6,450 km (4,000 mi) to the Gulf of Mexico. The eastern reaches of the great interior plain are bounded on the north by the Great Lakes, which are thought to contain about half the world's total supply of fresh water. Under US jurisdiction are 57,441 sq km (22,178 sq mi) of Lake Michigan, 54,696 sq km (21,118 sq mi) of Lake Superior, 23,245 sq km (8,975 sq mi) of Lake Huron, 12,955 sq km (5,002 sq mi) of Lake Erie, and 7,855 sq km (3,033 sq mi) of Lake Ontario. The five lakes are now accessible to oceangoing vessels from the Atlantic via the St. Lawrence Seaway. The basins of the Great Lakes were formed by the glacial ice cap that moved down over large parts of North America some 25,000 years ago. The glaciers also determined the direction of flow of the Missouri River and, it is believed, were responsible for carrying soil from what is now Canada down into the central agricultural basin of the United States. The great interior plain consists of two major subregions: the fertile Central Plains, extending from the Appalachian highlands to a line drawn approximately 480 km (300 mi) west of the Mississippi, broken by the Ozark Plateau; and the more arid Great Plains, extending from that line to the foothills of the Rocky Mountains. Although they appear flat, the Great Plains rise gradually from about 460 m (1,500 ft) to more than 1,500 m (5,000 ft) at their western extremity.
The Continental Divide, the Atlantic-Pacific watershed, runs along the crest of the Rocky Mountains. The Rockies and the ranges to the west are parts of the great system of young, rugged mountains, shaped like a gigantic spinal column, that runs along western North, Central, and South America from Alaska to Tierra del Fuego, Chile. In the continental United States, the series of western ranges, most of them paralleling the Pacific coast, are the Sierra Nevada, the Coast Ranges, the Cascade Range, and the Tehachapi and San Bernardino mountains. Between the Rockies and the Sierra Nevada–Cascade mountain barrier to the west lies the Great Basin, a group of vast arid plateaus containing most of the desert areas of the United States, in the south eroded by deep canyons. The coastal plains along the Pacific are narrow, and in many places the mountains plunge directly into the sea. The most extensive lowland near the west coast is the Great Valley of California, lying between the Sierra Nevada and the Coast Ranges. There are 71 peaks in these western ranges of the continental United States that rise to an altitude of 4,267 m (14,000 ft) or more, Mt. Whitney in California at 4,418 m (14,494 ft) being the highest. The greatest rivers of the Far West are the Colorado in the south, flowing into the Gulf of California, and the Columbia in the northwest, flowing to the Pacific. Each is more than 1,900 km (1,200 mi) long; both have been intensively developed to generate electric power, and both are important sources of irrigation.
Separated from the continental United States by Canadian territory, the state of Alaska occupies the extreme northwest portion of the North American continent. A series of precipitous mountain ranges separates the heavily indented Pacific coast on the south from Alaska's broad central basin, through which the Yukon River flows from Canada in the east to the Bering Sea in the west. The central basin is bounded on the north by the Brooks Range, which slopes down gradually to the Arctic Ocean. The Alaskan Peninsula and the Aleutian Islands, sweeping west far out to sea, consist of a chain of volcanoes, many still active. The state of Hawaii consists of a group of Pacific islands formed by volcanoes rising sharply from the ocean floor. The highest of these volcanoes, Mauna Loa, at 4,168 m (13,675 ft), is located on the largest of the islands, Hawaii, and is still active.
The lowest point in the United States is Death Valley in California, 86 m (282 ft) below sea level. At 6,194 m (20,320 ft), Mt. McKinley in Alaska is the highest peak in North America. These topographic extremes suggest the geological instability of the Pacific Coast region. Major earthquakes destroyed San Francisco in 1906 and Anchorage, Alaska, in 1964, and the San Andreas Fault in California still causes frequent earth tremors. Washington State's Mt. St. Helens erupted in 1980, spewing volcanic ash over much of the Northwest.
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