United States - officially United States of America, republic (2000 pop. 281,421,906), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. It consists of 50 states and a federal district. The conterminous (excluding Alaska and Hawaii) United States stretches across central North America from the Atlantic Ocean on the east to the Pacific Ocean on the west, and from Canada on the north to Mexico and the Gulf of Mexico on the south. The state of Alaska is located in extreme NW North America between the Arctic and Pacific oceans and is bordered by Canada on the east. The state of Hawaii, an island chain, is situated in the E central Pacific Ocean c.2,100 mi (3,400 km) SW of San Francisco. Washington, D.C., is the capital of the United States, and New York is its largest city.

The outlying territories and areas of the United States include: in the Caribbean Basin, Puerto Rico (a commonwealth associated with the United States) and the Virgin Islands of the United States (purchased from Denmark in 1917); in the Pacific Ocean, Guam (ceded by Spain after the Spanish-American War), the Northern Mariana Islands (a commonwealth associated with the United States), American Samoa, Wake Island, and several other islands. The United States also has compacts of free association with the Republic of the Marshall Islands, the Republic of Palau, and the Federated States of Micronesia.

Political Geography

The conterminous United States may be divided into several regions: the New England states ( Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut), the Middle Atlantic states ( New York, New Jersey, Pennsylvania, Delaware, Maryland, Virginia, and West Virginia), the Southeastern states ( North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Arkansas, Tennessee, and Kentucky), the states of the Midwest ( Ohio, Indiana, Illinois, Michigan, Wisconsin, Minnesota, Iowa, and Missouri), the Great Plains states ( North Dakota, South Dakota, Nebraska, and Kansas), the Mountain states ( Montana, Idaho, Wyoming, Colorado, and Utah), the Southwestern states ( Oklahoma, Texas, New Mexico, and Arizona), and the states of the Far West ( Washington, Oregon, California, and Nevada).

Alaska is the largest state in area (656,424 sq mi/1,700,578 sq km), and Rhode Island is the smallest (1,545 sq mi/4,003 sq km). California has the largest population (2000 pop. 33,871,648), while Wyoming has the fewest people (2000 pop. 493,782). In the late 20th cent., Nevada, Arizona, Florida, Colorado, Utah, Georgia, and Texas experienced the fastest rates of population growth, while California, Texas, Florida, Georgia, Arizona, and North Carolina gained the greatest number of residents. West Virginia, North Dakota, and the District of Columbia experienced population decreases over the same period. The largest U.S. cities are New York, Los Angeles, Chicago, Houston, and Philadelphia. Among the other major cities are Boston, Pittsburgh, Baltimore, Washington, D.C., Richmond, Virginia Beach, Charlotte, Atlanta, Jacksonville, Tampa, Miami, Cleveland, Columbus, Cincinnati, Detroit, Indianapolis, Milwaukee, Minneapolis, Saint Louis, Nashville, Memphis, New Orleans, Kansas City, Oklahoma City, Dallas– Fort Worth, Austin, San Antonio, El Paso, Albuquerque, Denver, Salt Lake City, Phoenix, Tucson, Las Vegas, Seattle, Portland, Sacramento, San Francisco, San Jose, Fresno, Long Beach, San Diego, and Honolulu.

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 East and the Gulf Coast

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.

The Plains and Highlands of the Interior

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.

The Western Mountains and Great Basin

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.

The Pacific Coast, Alaska, and Hawaii

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