Official name: United States of America
Area: 9,629,091 square kilometers (3,717,813 square miles)
Highest point on mainland: Mount McKinley (6,194 meters/20,322 feet)
Lowest point on land: Death Valley (86 meters/282 feet below sea level)
Hemispheres: Northern and Western
Time zones: Eastern, 7 A.M. = noon GMT; Central, 6 A.M. = noon GMT; Mountain, 5 A.M. = noon GMT; Pacific 4 A.M. = noon GMT; Yukon, 3 A.M. = noon GMT; Alaska and Hawaii, 2 A.M. = noon GMT; western Alaska, 1 A.M. = noon GMT.
Longest distances: 4,662 kilometers (2,897 miles) from east-northeast to west-southwest; 4,583 kilometers (2,848 miles) from south-southeast to north-northwest
Land boundaries: 12,219 kilometers (7,593 miles) total boundary length; Canada 8,893 kilometers (5,526 miles); Mexico 3,326 kilometers (2,067 miles)
Coastline: 19,924 kilometers (12,380 miles)
Territorial sea limits: 22 kilometers (12 nautical miles)
The United States of America, the world's third-largest country, occupies the central part of the North American continent, between Canada and Mexico, and also includes the states of Alaska, at the northwestern edge of the continent, and Hawaii, an island state in the Pacific Ocean. The United States is comprised of fifty states.
Except for Puerto Rico (see entry on Puerto Rico), U.S. territories and dependencies consist of very small islands. The U.S. Virgin Islands and Navassa Island are located in the Caribbean; the rest are in the Pacific Ocean. These include American Samoa, Guam, Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Islands, Palmyra Atoll, and Wake Island.
Although the continental (forty-eight contiguous) United States lie within the Northern Hemisphere's temperate zone, there are wide variations in climate, including extremes in temperature and violent weather disturbances. The states along the eastern seaboard have a continental climate despite their proximity to the Atlantic Ocean, as do the states of the Midwest. The mean annual temperature in Miami, Florida, is 24°C (76°F), while that in Boston, Massachusetts, is 11°C (51°F). In the country's vast central lowlands, there is an even greater tendency toward sharp contrasts and sudden changes. The northern Great Plains has seen summer highs of 49°C (121°F) and winter lows of -51°C (-60°F).
In contrast to the continental climate experienced in much of the country, the U.S. West Coast, with its proximity to the Pacific Ocean, has a maritime climate with warm summers and mild winters. Seattle, Washington, on the northern part of the Pacific coast, has average temperatures of 4°C (39°F) in January and 18°C (65°F) in July. Farther south along the coast, Los Angeles, California, averages 13°C (56°F) in January and 21°C (69°F) in July.
The panhandle region of southern Alaska has a mild maritime climate, while the interior of the state has extremes of both heat and cold. The far north, within the Arctic Circle, has a uniformly frigid arctic climate. By contrast, Hawaii has a stable, even climate with temperatures averaging 23°C (73°F) in January and 27°C (80°F) in July.
Average annual rainfall is more than 100 centimeters (40 inches) in an area covering roughly the eastern two-fifths of the country. The prairie and Great Plains states to the north and west are considerably drier, however, with average rainfall as low as 46 centimeters (18 inches) per year, dropping to 25 centimeters (10 inches) in the northern plains. In the Rocky Mountains, precipitation varies according to altitude, with the higher elevations receiving more rain. The deserts to the west of the Rockies are the driest parts of the country, with unevenly distributed precipitation in the region ranging from annual averages of 8 centimeters (3 inches) in Yuma, Arizona, to as much as 152 centimeters (60 inches) in central Idaho and Washington State. Annual rainfall in the Pacific coastal area varies widely with latitude, from 4.5 centimeters (1. 8 inches) in Death Valley to more than 356 centimeters (140 inches) in the Olympic Mountains of Washington State.
In Alaska, the panhandle and the southern arc of the Aleutian Islands have a wet maritime climate, while the interior is, on the whole, quite dry, despite its snow. Hawaii is generally moderately rainy (71 centimeters/28 inches annually), with very heavy rainfall occurring at higher elevations.
In its broadest topographic outline, the continental United States comprises a large, central lowland—accounting for close to half its total area—bordered on the east and west by highlands. The western highland area, which begins with the Rocky Mountains, is by far the more extensive of the two, accounting for about one-third of the total area of the country. The band of highlands on the east, which is lower and less extensive, consists of the Appalachian Mountains. The Mississippi River and its tributaries dominate the lowland in between, with the Great Lakes to the north. The western part of this lowland is known as the Great Plains. East and south of the Appalachian Mountains are coastal plains.
Alaska has seven topographical regions: the southeastern coastal mountains; the glaciered coast; south-central Alaska; the Alaska Peninsula and the Aleutian Islands; interior Alaska; the Seward Peninsula and the Bering Coast Uplands; and the Arctic Slope. The Hawaiian Islands are basaltic volcanoes near the middle of the Pacific Ocean along a northwest-trending ridge.
The continental United States are bordered on the east by the Atlantic Ocean, on the southeast by the Gulf of Mexico, and on the west by the Pacific Ocean. Alaska lies amidst several bodies of water: on the north are the Beaufort Sea and the Arctic Ocean; on the west are the Chukchi Sea, the Bering Strait, and the Bering Sea; and on the south is the Gulf of Alaska. The Hawaiian Islands lie in the North Pacific Ocean.
Off the Atlantic coast, the continental shelf is more than 161 kilometers (100 miles) wide; beyond this, the ocean floor plunges to depths of more than 3.2 kilometers (2 miles). The continental shelf along most of the Pacific coast is quite narrow. Two major mountain ridges extend about 2,414 kilometers (1,500 miles) westward from the coast into the Pacific Ocean. Oceanic trenches line the southern shores of the Alaska Peninsula and the Aleutian Islands. The Aleutian Trench, at 7,620 meters (25,000 feet) below sea level, is the lowest point bordering North America.
The major inlets on the Atlantic coast are Long Island Sound and the Delaware and Chesapeake Bays to the north, and Albemarle and Pamlico Sounds farther south, in the Sea Islands area. The Straits of Florida separate the state of Florida from the Bahamas to the southeast.
Puget Sound in the state of Washington is the major inlet on the Pacific coast; the Strait of Juan de Fuca connects Puget Sound to the ocean. The Sacramento River in northern California drains into the Pacific at San Francisco Bay.
There are few large islands off the coast of the continental United States. Long Island, near the mouth of the Hudson River, is the largest of these (4,462 square kilometers/1,723 square miles). The Florida Keys are a series of small islands arcing southwest from the south coast of Florida into the Gulf of Mexico. There are numerous smaller islands in Chesapeake Bay, the Outer Banks, and off the northeastern coast. The largest islands off the Pacific coast are the Santa Barbara Islands, and the only other islands of any significant size are found in Puget Sound.
The state of Hawaii consists of five large islands—Hawaii, Maui, Molokai, Oahu, and Kauai—with four smaller islands close by—Kahoolawe, Lanai, Kaula, and Niihau. Hawaii itself is the most easterly and largest of the islands, with an area of 10,414 square kilometers (4,021 square miles). There are many islands located off the Alaskan coast. The southern part of Alaska is the site of the coastal Alexander Archipelago. Further east is the Alaska Peninsula, with the Aleutian Islands extending from its tip.
The Atlantic coastline can be divided into three sections. Large peninsulas characterize the northern (or embayed) section, which stretches from the northeastern end of the country halfway down the coast to Chesapeake Bay. South of the embayed section is the Sea Islands section, a region of coastal lagoons and islands. The Outer Banks are the most famous and extensive of these. The final segment of the eastern coast is the smooth, sandy, eastern coast of the Florida Peninsula. The southern, or Gulf, coast has multiple indentations in its eastern section, including Tampa and Mobile Bays. The irregularly shaped Mississippi Delta juts out in the middle, and the shoreline to the west of the river is smoother.
The Pacific shoreline is straight and fully exposed to the surf, without barrier beaches or lagoons. There are two major indentations in the Pacific coast: Puget Sound and San Francisco Bay. The coast of Alaska is deeply embayed to the west, southwest, and south. Point Barrow on the Arctic Ocean coast is the northernmost point in the country.
The northern New England coast is rocky, while the Atlantic coast south of New England is a plain with extensive sandy beaches. In the Sea Islands section of the Atlantic coastal plain, the islands off the coast have attractive sandy beaches facing the ocean. Sandy beaches also rim much of the Gulf Coast, except for the Mississippi Delta area, where marshes, swamps, and bayous extend to the sea. Much of the Pacific coast is mountainous, and a narrow coastal plain rings Puget Sound. Alaska's coast is mostly low-lying in the north and west and mostly mountainous in the south and in both panhandles. The Hawaiian Islands are ringed with mostly narrow coastal plains.
The five Great Lakes make up the world's largest group of freshwater lakes, and Lake Superior has the greatest surface area of any freshwater lake on Earth (82,362 square kilometers/31,800 square miles). Outside of the Great Lakes, the next largest body of water in the country is the much smaller Great Salt Lake in Utah, with an area of about 5,957 square kilometers (2,300 square miles). Other major lakes in the western United States include Sevier Lake, Crater Lake, Lake Tahoe, and Yosemite Lake. Florida and Minnesota are also known for their many lakes. New York is home to the Finger Lakes and Lake Champlain. The Okefenokee Swamp in northeastern Florida, with an area of around 1,813 square kilometers (700 square miles), is the largest single swamp in North America. Occupying the tip of the Florida peninsula, south of Lake Okeechobee, is the vast network of swamps and marshes known as the Everglades.
With few exceptions, the rivers to the east of the Continental Divide drain into the Atlantic Ocean and the Gulf of Mexico; those to the west drain into the Pacific. There are many short rivers east of the Appalachian Mountains that flow into the Atlantic; chief among them is the Hudson River. Even the longest of these waterways flows for only several hundred miles, however.
The Mississippi River and its tributaries drain most of the central United States. The Mississippi is one of the world's great rivers in terms of both volume and length (4,127 kilometers/2,348 miles). It flows south across the country, somewhat east of its center, and empties into the Gulf of Mexico in a great delta. The Arkansas, Ohio, and Missouri Rivers are three of its most important tributaries. The Missouri is the longest river in the country, as well as the longest on the continent of North America.
The principal river of the Colorado Plateau is the Colorado River (2,350 kilometers/1,450 miles). The Colorado flows southwest and receives all of the other large rivers in the region, including the Green, San Juan, and Gila, before traveling into Mexico and eventually emptying into the Pacific Ocean. The Columbia River and its large tributary, the Snake River, dominates the Columbia Plateau. In Alaska, the Yukon is the state's longest river (3,185 kilometers/1,979 miles).
The Great Basin is a vast area in the western United States that has no drainage to the ocean. Arid areas of bare rock characterize both the Great Basin and the Colorado Plateau, with sparse vegetation elsewhere. Included in this region are the Great Salt Lake Desert, Death Valley, and the Sonoran Desert region, which extends southward into Mexico and also includes the Mojave Desert in southern California. The true deserts of the southwest support only scrub and a few annuals that appear intermittently, after it rains.
Yellowstone National Park is thought to contain roughly ten thousand hot springs and geysers, more than half of all such features on Earth.
A low-lying coastal plain extends for more than 3,200 kilometers (2,000 miles) along the eastern and southeastern fringes of the country, encompassing the coasts of both the Atlantic Ocean and the Gulf of Mexico. The plain is narrow in New England but reaches a maximum width of about 320 kilometers (200 miles) farther south. Between the Rocky Mountains and the Appalachian Mountains is a vast area of grasslands and plains, which extends from the coastal plains of the south well into northern Canada. West of the Mississippi, the rolling prairies are known as the Great Plains. Other extensive plains occur in the structural basins of the western mountains. Death Valley, the lowest point in North America, lies along the southwestern edge of the Great Basin. There are foothills associated with all of the major mountain ranges of the United States. The Black Hills in the northern Great Plains are actually dome mountains; some summits rise to elevations of over 610 meters (2,000 feet).
Mountains cover one-quarter of the country. The central plains are flanked on the east by the Appalachian Mountains and on the west by the Rocky Mountain system. More high mountains can be found along the Pacific coast, as well as in Alaska and Hawaii.
The Appalachians are the major mountain range in the eastern United States. Although they are neither as high nor as rugged as the Rocky Mountains, they are very extensive. They enter the United States from Canada in the northeast and extend southwest most of the way to the Gulf of Mexico. Their highest peak is Mount Mitchell (2,037 meters/6,684 feet). The Appalachian Highlands consist of several distinct ranges, including the Great Smoky Mountains, the Blue Ridge Mountains, and the Allegheny Mountains. The Adirondack Mountains in northern New York State have a domelike structure.
The Southern Rockies, the highest section of the Rocky Mountains, contain many peaks of elevations over 4,267 meters (14,000 feet). Among the Southern Rockies are the Laramie, San Juan, and Sacramento Mountains, as well as the highest peak in the Rocky Mountains: Mt. Elbert (4,399 meters/14,433 feet). Major ranges in the Middle Rockies include the Bighorn, Absaroka, Wind River, Uinta, and Wasatch. The Northern Rockies, the lowest part of the chain, contain the Bitterroot and Lewis ranges.
The Pacific mountain system parallels, and in some places extends to, the western coast, covering an area of about 518,000 square kilometers (200,000 square miles). It includes several different mountain ranges, as well as valleys and deserts. The major divisions of the Pacific mountain system are the Cascade and Sierra Mountains, the Coast Ranges, and the Lower California Peninsular Range. Mount Whitney, in the Sierra Nevada, is the highest point in the United States outside of Alaska, at 4,418 meters (14,495 feet).
The Coast Ranges, a series of mountains along the Pacific coastline, contain summits that do not exceed 1,219 meters (4,000 feet). The Lower California Peninsular Range, located mostly in Mexico, extends across the border into the extreme southwest of the United States.
Alaska's major mountain ranges are found in the south-central part of the state. The north and south peaks of Mt. McKinley (Denali), at 6,194 meters (20,322 feet) and 5,934 meters (19,470 feet), respectively, are the highest peaks on the North American continent. Lower mountains are found in the northern part of the state.
The Hawaiian Islands in the Pacific Ocean all are volcanic in origin and thus have mountainous interiors.
The island of Hawaii is the site of two great volcanoes, Mauna Kea and Mauna Loa, which erupt frequently but gently. More than a dozen volcanoes in the western part of the continental United States have been classified as potentially active by the U.S. Geological Survey, as well as seven in Hawaii and many more along the Alaskan coastline and on neighboring islands. The most recent volcanic eruption in the lower forty-eight states was that of Mount St. Helens in Washington State in 1980; the unexpected explosion killed more than sixty people.
The country's most dramatic canyons are in the Intermontane Region between the Rocky Mountains and the Pacific coastal mountains, carved by the major rivers located here. In the Colorado Plateau, the Colorado River passes through the picturesque Grand Canyon, which is more than 1,600 meters (5,280 feet) deep and 349 kilometers (217 miles) long. Farther north are the Canyonlands of southeastern Utah; the pinnacles and spires of red rock in southwestern Utah's Bryce Canyon are among the most remarkable sights in the country. In the Columbia Plateau is found the single deepest canyon in the United States: Hell's Canyon. Its average depth is 2,000 meters (6,600 feet), and it extends for 200 kilometers (125 miles).
Mammoth Cave in Kentucky is the world's largest cave system, with 563 kilometers (350 miles) of caves mapped out to date. Another of the nation's major cave sites is the Carlsbad Caverns National Park in New Mexico, home to some one hundred caves.
About a quarter of the country rests on plateaus. The eight major ones are the Piedmont, Appalachian, and interior low plateaus in the east; the Ozark Plateau, Edwards Plateau, and the Llano Estacado in the central U.S.A.; and the Colorado and Columbia Plateaus, both of which belong to the Intermontane Plateau region of the west. The Colorado Plateau—between the Southern Rockies, the Sierra Nevada, and the Great Basin—is the most colorful part of the United States, with spectacular geological features, including volcanoes, mesas, and dome mountains. The Grand Canyon is located in the southwestern part of this region. Yellowstone National Park is located on the Yellowstone Plateau east of the Snake River plain, at the southeastern edge of the Columbia Plateau.
The Mount Rushmore National Memorial in South Dakota contains sculptures of four U.S. presidents (George Washington, Thomas Jefferson, Abraham Lincoln, and Theodore Roosevelt), each of which is 18 meters (60 feet) tall, carved into the granite face of a peak that rises to 1,707 meters (5,600 feet). Major bridges in the United States include the Brooklyn Bridge and the Verrazano Narrows and George Washington Bridges, all in New York City, and the Golden Gate and San Francisco-Oakland Bay Bridges in San Francisco.
The New York State Barge Canal, a modification and extension of the former Erie Canal, links the Hudson River and Lake Champlain to the Great Lakes. Both the Colorado and Columbia Rivers in the western United States feed into major reservoirs, including Franklin D. Roosevelt Lake behind Grand Coulee Dam. The Grand Coulee Dam is the country's largest hydroelectric project, and Lake Mead is its largest man-made reservoir.
Brinkley, Douglas. The Magic Bus: An American Odyssey . New York: Anchor Books, 1994.
Heat Moon, William Least. Blue Highways: A Journey into America . Boston: Little, Brown, 1982.
McPhee, John A. Coming into the Country. New York: Farrar, Straus & Giroux, 1977.
Raban, Jonathan. Old Glory: An American Voyage . New York: Simon and Schuster, 1981.