United States of America

• Area: 3,717,813 sq mi (9,629,091 sq km) / World Rank: 4
• Location: Northern and Western Hemispheres, bordering Canada to the north, Mexico to the south, the Atlantic Ocean to the east, the Gulf of Mexico to the southeast, and the Pacific Ocean to the west. The exclave of Alaska borders Canada in the east.
• Coordinates: 38°00′N, 97°00′W
• Borders: 7,593 mi (12,219 km) / Canada, 5,526 mi (8,893 km); Mexico, 2,067 mi (3,326 km)
• Coastline: 12,380 mi (19,924 km)
• Territorial Seas: 12 NM
• Highest Point: Mt. McKinley, 20,322 ft (6,194 m)
• Lowest Point: Death Valley, 282 ft (86 m) below sea level
• Longest Distances: 2,897 mi (4,662 km) ENE-WSW /2,848 mi (4,583 km) SSE-NNW
• Longest River: Missouri, 2,466 mi (3,968 km)
• Largest Lake: Lake Superior, 31,820 sq mi (82,732 sq km)
• Natural Hazards: Floods; hurricanes; tornadoes; forest fires; earthquakes
• Population: 278,058,881 (July 2001 est.) / World Rank: 3
• Capital City: Washington, D.C., mid-Atlantic coast
• Largest City: New York City, northern Atlantic coast, 8,008,278 (2000 census)

OVERVIEW

The United States of America (United States, U.S.A., U.S.), the world's third-largest country, occupies the central part of the North American continent, between Canada and Mexico. It extends about 1,200 mi (1,900 km) north to south between these two countries and spans the width of the continent from the Atlantic Ocean to the Pacific. In addition to the forty-eight conterminous states (and the District of Columbia) contained within this area, Alaska, at the northwestern edge of the continent, and Hawaii, an island state in the Pacific Ocean about 2,500 mi (4,000 km) west of North America, are integral parts of the U.S.A. It is the second largest country on the continent, Canada being larger. The United States also has many overseas dependencies and possessions. Most notable among these are Puerto Rico and the U.S. Virgin Islands in the Caribbean Sea; and Guam, American Samoa, and the North Mariana Islands, in the Pacific Ocean.

In its broadest topographic outline, the conterminous U.S. 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 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 lowland between them is dominated by the Mississippi River and its tributaries, 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.

The conterminous United States can be broken down into the following major geographic regions:

A low-lying coastal plain extends for more than 2,000 mi (3,200 km) 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 200 mi (320 km) farther south. Its terrain is mostly flat but includes ridges and low hills, as well as large tracts of marshland.

The Appalachian Mountains extends northeast to southwest from Maine to Alabama, a length of some 1,200 mi (1,000 km). This varied region includes mountains, plateaus, and valleys. Among its subregions are the Piedmont Plateau, the Blue Ridge Mountains, the Appalachian Plateaus, the Adirondacks, and the St. Lawrence Valley. The area of this region is roughly equal to that of the Atlantic coastal plain.

The central lowland, with an area of about 585,000 sq mi (1,515,000 sq km), is the country's agricultural heart-land. Stretching from the Appalachians to the Great Plains, and from the northern border to the southern section of the coastal plain, it is the country's largest region, accounting for about 15 percent of its total area and extending over 16 states. This region, which includes the Great Lakes, ranges in elevation from under 1,000 ft (305 m) to around 2,000 ft (600 m), gradually getting higher from east to west and encompassing flat plains, rolling land, woodlands, and valleys.

Extending northward from central Texas to the Canadian border, in an unbroken band with an average width of about 500 mi (805 km), the Great Plains represent a continuation of the central lowland. Continuing that region's gradual westward rise in elevation, from 2,000 ft (600 m) to over 5,000 ft (1,500 m), they cover an area of approximately 450,000 sq mi (1,165,000 sq km). The Great Plains include such varied subregions as the Missouri Plateau, the Black Hills, the High Plains, the Colorado Piedmont, and the Llano Estacado.

West of the Great Plains are the Rocky Mountains. They are characterized by many high peaks, rugged relief, extensive forests, and spectacular scenery. This is the principal mountain range of North America, extending across the continent from north to south in a vast series of parallel ridges. They are part of a continuous chain of mountains stretching from Alaska to Patagonia, the backbone of the Western Hemisphere. Within the United States they constitute the Continental Divide, with rivers to their east feeding into the Atlantic and the Gulf of Mexico, rivers to their west emptying into the Pacific, or never reaching the sea at all. Together with the Wyoming Basin they cover about 10 percent of the country.

The Intermontane Plateau region, which lies between the Rocky Mountains and the Pacific coastal mountains, covers the largest area within the western highland region: about 550,000 sq mi (1,424,000 sq km). It encompasses vast expanses of desert, as well as river basins, valleys, mountains, canyons, and hills. Much of this region has no drainage to the sea.

The Pacific mountain system parallels, and in some places extends to, the western coast, covering an area of about 200,000 sq mi (518,000 sq km) and including several different mountain ranges as well as a variety of other physical features, such as valleys and deserts.

Clearly demarcated among the foregoing large regions are two smaller but topographically distinct ones. The Superior Upland, an area bordering Lake Superior and extending 75,000 sq mi (194,000 sq km) over parts of Minnesota, Wisconsin, and Michigan, is a continuation of the Precambrian Shield of Canada that rises to elevations of between 1,000 and 2,000 ft (305 and 610km). The Interior Highlands, which include the Ozark Plateaus and the Ouachita Mountain region, have average elevations as high as 2,600 ft (792 m). With an area of less than 100,000 sq mi (250,000 sq km), they include parts of Missouri, Arkansas, and Oklahoma.

Separated from the rest of the country by Canada, Alaska is the largest state, with a land area about one-fifth that of the other forty-nine states combined. It is characterized by extremes of elevation, from Mount McKinley, at 20,320 ft (6,193 m) the highest point in North America, to the Aleutian Trench just off shore, at 25,000 ft (7,620 m) below sea level the lowest point bordering North America. 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 that divides two oceanic deeps, each of which descends more than 18,000 ft (5,486 m) below sea level. The United States also has a number of overseas island territories and dependencies in the Caribbean Sea and the Pacific Ocean, covering a total area of approximately 4,000 sq mi (10,360 sq km). Puerto Rico is the largest of these.

All of the United States lies on the North American Tectonic Plate, except for a narrow strip in southern California and the Hawaiian Islands, which are on the Pacific Plate. The Juan de Fuca Plate is located not far off the Pacific coast. Certain regions of the United States are subject to either severe or moderate seismic activity. One seismic belt, which extends along parts of the Rocky Mountains, has been the site of two major earthquakes, one at Helena, Montana, in 1925 and the other at Hebgen Lake in 1959. Earthquakes are more frequent on the Pacific Coast, along fault lines in the area where the North American and Pacific Tectonic Plates meet. The Pacific coastal area experienced more than 200 earthquakes between the mid-nineteenth and mid-twentieth century, including highly destructive ones in 1857, 1872 and 1906. Earthquakes are also frequent in two belts of seismic activity in Alaska. Volcanoes can be found all along the Pacific coast and in Alaska and Hawaii.

MOUNTAINS AND HILLS

Mountains make up 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, and in Alaska and Hawaii.

The Rocky Mountains

The Southern Rockies are the highest in the chain, with many peaks of over 14,000 ft (4,267 m). They can be crossed only through high passes, all above 9,000 ft (2,743 m). These mountains consist of a series of ranges, among them the Laramie, Front, Sangre de Cristo, San Juan, and Sacramento Mountains. The highest peak in the Rocky Mountains is Mt. Elbert (14,433 ft / 4,399 m) in the Southern Rockies. The Middle Rockies include some of the most impressive mountains in the western United States, but only some parts are higher than 11,000 ft (3,353 m). Major ranges include the Bighorn, Absaroka, Wind River, Uinta and Wasatch. The Northern Rockies are the lowest part of the chain, rarely exceeding 11,000 ft (3,353 m). The Bitterroot and Lewis Ranges are the largest in this part of the Rockies.

The Appalachian Mountains

The Appalachians are the major mountain range of the eastern U.S.A. Although they are neither as high nor as rugged as the Rocky Mountains, with more rounded landforms, they are very extensive and were a serious impediment to travel in the early history of the country. They enter the United States from Canada in the northeast and extend southwest most of the way to the Gulf of Mexico. Only 15 peaks exceed 2,400 ft (731 m), with the highest being Mt. Mitchell (6,684 ft / 2,037 m).

The Appalachian Highlands consist of several distinct ranges. The Great Smoky Mountains in the southern part of the Appalachians are a national park. The Blue Ridge Mountains extend from Georgia to Pennsylvania. In places they consist of a single ridge; elsewhere, a complex of closely spaced ridges. The Allegheny Mountains are parallel to the Blue Ridge. The Adirondack Mountains in northern New York State have a domelike structure more than 100 mi (161 km) in diameter. The Appalachians are at their highest average elevation in New England, particularly the Green Mountains in Vermont, the Hoosac Mountains in Massachusetts, and the White Mountains of New Hampshire and Maine. Isolated mountains that rise above the general level of the surrounding terrain are called monadnocks.

Pacific Mountains

The Pacific mountain system comprises some 200,000 sq mi (518,135 sq km) along the western coast of the U.S. It includes granitic mountains, such as the Sierra Nevada and Klamath mountains; volcanic mountains such as the Cascade Range; folded and faulted mountains such as the Transverse Ranges, the Olympic Mountains, and the Oregon Coast Range; and dome mountains, such as the Marysville Buttes of the California Central Valley. These mountains are among the highest, roughest, and most scenic in the United States. The major divisions of the Pacific mountain system are the Cascade and Sierra Mountains, the Coast Ranges, and the Lower California Peninsular Range.

The Sierra Nevada is a huge block mountain about 350 mi (563 km) long and roughly 60 mi (97 km) wide. Not a single river crosses it in its entire length. The Cascades extend north from the Sierra Nevada all the way to Canada. These high mountains form the western borders of the great plateaus of the western U.S., with the Rocky Mountains in the east. Mt. Whitney, in the Sierra Nevada, is the highest point in the United States outside of Alaska at 14,495 ft (4,418 m).

The Coast Ranges are a series of mountains along the Pacific coastline, including the California coast ranges, the Transverse ranges, the Klamath Mountains, the Oregon coast range, and the Olympic Mountains in Washington. The highest ranges in the Transverse Ranges are the San Gabriel and the San Bernardino Mountains, both located along the San Andreas fault. The Oregon Coast Range consists of irregular hills and low mountains along the coast of Oregon and southwestern Washington. The highest summits are lower than 4,000 ft (1,219 m). The Lower California Peninsular Range is located mostly in Mexico, but 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: Chugach, Wrangell, Talkeetna, the Alaska Range, and the Aleutian Range. The highest mountains in the U.S. are all in Alaska. The north and south peaks of Mt. McKinley (Denali), at 20,320 ft (6,193 m) and 19,470 ft (5,934 m), respectively, are the highest peaks on the North American continent. Lower mountains, the Brooks Range, are found in the northern part of Alaska.

The Hawaiian Islands in the Pacific Ocean are all volcanic in origin and have mountainous interiors. The island of Hawaii itself is the site of two great volcanoes, Mauna Kea (13,796 ft / 4,197 m) and Mauna Loa (13,680 ft / 4,170 m).

Plateaus

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 Piedmont stretches along the eastern edge of the Appalachian Mountains south from the Hudson River to the end of the range. It has a maximum width of about 125 mi (201 km). Altitudes range between 100 and 1,000 ft (30 and 305 m). The Appalachian Plateaus are a series of plateaus west of the Appalachian Mountains, the most famous of which is the Cumberland Plateau. They too begin near the Hudson River and run south along the mountains, with their highest elevations, roughly 3,000 ft (914 m), near the Hudson. The interior low plateaus are another series of plateaus, found to the west of the Appalachian Plateaus. They are less well defined and have lower elevations than the Appalachian Plateaus, and run parallel to the Appalachians for 600 mi (965 km) from Alabama to Ohio

The Ozark Plateaus, often called the Ozarks, are found southwest of the junction of the Mississippi and Missouri Rivers in the south central United States. They are sometimes referred to as mountains, but for most of their area have average elevations between only 1,000 and 1,500 ft (300 and 460 m). The Edwards Plateau and Llano Estacado are both elevated areas of the Great Plains found along the southeastern edge of the Rocky Mountains.

The Colorado Plateau, between the Southern Rockies, Sierra Nevada, and the Great Basin, is the most colorful part of the United States, with spectacular geological features. It is primarily semi-arid, with the Sonoran Desert bordering it to the south. The general plateau surface is higher than 5,000 ft (1,524 m), and some peaks reach 11,000 ft (3,353 m). Striking topographic features include volcanoes, cinder cones, volcanic necks, mesas, and dome mountains. The Grand Canyon is located in the southwestern part of this region.

The Columbia Plateau is located on the far side of the Great Basin from the Colorado Plateau, in the north. It too is characterized by a semiarid climate and dry canyons, and is dominated by the Columbia and Snake Rivers and their plains. Yellowstone National Park is located on the Yellowstone Plateau east of the Snake River Plain, at the southeastern edge of the Columbia Plateau, where volcanic activity is still strong.

Canyons and Depressions

The country's most dramatic canyons are in the Intermontane Region between the Rocky Mountains and the Pacific coastal mountains. The major rivers of these regions have carved deep canyons in the plateaus.

In the Colorado Plateau, the Colorado River passes through the picturesque Grand Canyon, one of the country's most famous natural features. It is more than 1 mi (1,600 m) deep and 217 mi (349 km) long. Farther north in the same plateau region is the Canyonlands area of southeastern Utah, which is dominated by multiple canyons, some of which are more than 2,000 ft (600 m) deep. 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 the single deepest canyon in the United States: Hells Canyon, carved by the Snake River. Its average depth is 6,600 ft (2,000 m), and it extends for 125 mi (200 km). In the same region, the Columbia River has carved canyons that reach depths of 2,000 ft (600 m).

The Great Basin is a vast area in the western U.S. that has no drainage to the ocean. This is because, although it is well above sea level in most places, the surrounding mountains and plateaus are higher still. The Great Salt Lake is found in the northeastern part of the Great Basin. Death Valley, the lowest point in North America, is along the southwestern edge of the Great Basin. Much of the Basin is desert.

Hills and Badlands

There are foothills associated with all of the major mountain ranges of the United States. The best-known region outside of these mountains designated as hilly are the Black Hills of the northern Great Plains. These are actually dome mountains; some rise to elevations of over 2,000 ft (610 m). The craggy, heavily eroded peaks are forested at lower elevations but bare at the top, with steeply rising pinnacles and other rock formations. East of the Black Hills are the White River Badlands, where erosion has sculpted buttes, spires, and other fantastic shapes into the plateau. The badlands continue to erode, on average one inch per year.

INLAND WATERWAYS

Lakes

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, (31,800 sq mi / 82,362 sq km). Lake Huron is the next largest. Lake Michigan, the third largest, is the only one of the Great Lakes found entirely within the United States, the others are shared with Canada. It has an area of 22,178 sq mi (57,441 sq km) and a maximum depth of 923 ft (281 m). All three of these lakes drain into Lake Erie, which in turn empties into Lake Ontario. The St. Lawrence River flows northwest out of Lake Ontario into the Atlantic Ocean.

Outside of the Great Lakes, the next largest in the country is the much smaller Great Salt Lake in Utah, with an area of about 2,300 sq mi (5,957 sq km) and a maximum depth of 15 m (48 ft). Surprisingly, the arid western United States is a land of lakes, both dry and perennial, including not only the Great Salt Lake, but also Sevier Lake, Utah Lake, and Carson Sink in the Great Basin; and Lake Chelan, Crater Lake, Lake Tahoe, and Yosemite Lake in the mountains.

Florida has many lakes; Lake Okeechobee, to the north of the Everglades, covers 1,943 sq km (750 sq mi). Minnesota is also known for its lakes, including the Red Lakes, and also Lake of the Woods and Rainy Lake on the Canadian border. New York is home to the Finger Lakes, as well as Lake Champlain. The so-called Salton Sea is a salt lake in southern California that was formed unintentionally by run-off water from the Colorado River and from farming.

Rivers

With few exceptions, the rivers to the east of the Rocky Mountains drain into the Atlantic Ocean and the Gulf of Mexico; those to the west drain into the Pacific. There are a few rivers in the Great Basin that never reach any ocean, the largest of which is the Humboldt. The Red River of the North in the northern Great Plains drains north into Canada.

There are many short rivers east of the Appalachian Mountains that flow into the Atlantic. Even the longest of these flows for only several hundred miles. Chief among them is the Hudson River, which is linked via the Erie Canal to the Great Lakes. The Connecticut, Roanoke, and Savannah are other notable rivers of this region.

Most of the central United States drained by the Mississippi River and its tributaries. The Mississippi is one of the world's great rivers in terms of both volume and length (2,348 mi/4,127 km). It flows south across the country, somewhat east of its center, and empties into the Gulf of Mexico in a great delta, The most important of its tributaries are the Arkansas, Ohio, and Missouri Rivers. The Arkansas (1,450 mi / 2,333 mi) flows east from the Rocky Mountains across the center of the country, cutting through the Ozarks. It too has many tributaries, including the Cimarron and the Canadian Rivers. The Ohio River (975 mi / 1,569 km) forms at the confluence of three smaller rivers at Pittsburgh, in the Appalachians. It then flows west, and gradually south before merging with the Mississippi. Its largest tributary, the Tennessee River (652 mi / 1,049 km), is one of the most important in the eastern United States in its own right, with many hydroelectric dams and reservoirs along its course.

The largest of the Mississippi's tributaries by far is the Missouri, which at 2,466 mi (3,968 km) is longer than the Mississippi itself. The Missouri is the longest river in the country and the second-longest in North America. It has its source in the Northern Rockies, and flows east across the northern Great Plains before making a great curve south. It then flows southeast and meets the Mississippi near the middle of that river's course. The length of the Mississipppi-Missouri system from the source of the Missouri to the mouth of the Mississippi is 3,860 mi (6,211 km) making it the world's third-longest river system. The Missouri's many tributaries include the Bighorn, James, and Platte.

The rivers of the southern Great Plains are among the few in the central United States that reach the Gulf of Mexico directly, without flowing into the Mississippi first. The Rio Grande, along the border with Mexico, is one of the longest in the country at 1,885 ft (3,033 m). Its major tributary is the Pecos. At the far side of the central plains, the St. Lawrence River flows northeast out of Lake Ontario and into Canada.

The principal river of the Colorado Plateau is the Colorado River (1,450 mi / 2,350 km) itself. The Colorado flows southwest, and receives all of the other large rivers in the region, including the Green, San Juan, and Gila, before exiting into Mexico and eventually the Pacific Ocean. The Colorado has been dammed in several locations, forming the great Lake Mead and Lake Powell reservoirs. It also flows through the famous Grand Canyon. The Columbia Plateau is dominated by the Columbia River and its large tributary, the Snake. As with the Colorado, both rivers have dug deep canyons, and are the site of major reservoirs, including Franklin D. Roosevelt Lake behind Grand Coulee Dam. The Sacramento, Klamath, and Willamette are wide rivers of the Pacific mountain region, although they are short and drain small areas.

In Alaska the Yukon is the longest river (1,979 mi / 3,185 km). It is navigable almost all of the way to its headwaters in Canada. Its principal tributaries are the Koyukuk, Tanana, and Porcupine rivers. The Kuskokwim, Alaska's other major river, rises in the Alaska Range and flows southwest.

Wetlands

The Atlantic coastal plain, more than half of which is below 500 ft (152 m), is noted for its low topographic relief and extensive marshy tracts. Marshes begin to appear in some areas as the plain widens between New York City and North Carolina. Farther south, there is a network of lagoons, sea channels, and salt marshes between the shoreline of the Carolinas and Georgia, as well as the Great Dismal Swamp in North Carolina and Virginia.

The Okefenokee Swamp in northeastern Florida, with an area of around 700 sq mi (1,813 sq km), 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. Near the middle of the Gulf Coast shoreline is are the swamplands, mud flats, and bayous of the Mississippi River Delta. There are also swamplands in Upper Michigan and numerous large swamps in the deltas of Alaska's lower Yukon and lower Kuskokwim rivers.

THE COAST, ISLANDS, AND THE OCEAN

Oceans and Seas

The conterminous United States is bordered on the east by the Atlantic Ocean, on the southeast by the Gulf of Mexico, and on the west by the Pacific Ocean.

Off the Atlantic coast the continental shelf is more than 100 mi (161 km) wide; beyond this, the ocean floor plunges to depths of more than 2 mi (3.2 km).

The continental shelf along most of the Pacific coast is quite narrow. North of Point Conception it is barely 50 mi (80 km) wide, but south of this point the width increases to 150 mi (241 km). Two major mountain ridges extend about 1,500 mi (2,414 km) westward from the coast into the Pacific Ocean.

Alaska is bordered on the north by the Beaufort Sea and the Arctic Ocean, on the west by the Chukchi Sea, the Bering Strait, and the Bering Sea, and on the south by the Gulf of Alaska. The southern shores of the Alaska Peninsula and the Aleutian Islands are bordered by oceanic trenches. The Hawaiian Islands lie in the North Pacific Ocean.

Major Islands

There are few large islands off the coast of the conterminous United States. Long Island near the mouth of the Hudson River is the largest (1,723 sq mi / 4,462 sq km). 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, with the only other islands of any size found in Puget Sound. Isle Royale is a large island and nature preserve in Lake Superior.

The Hawaiian Islands are basaltic volcanoes near the middle of the Pacific Ocean along a northwest-trending ridge that divides two oceanic deeps, each of which descends more than 18,000 ft (5,486 m) below sea level. There are five large islands—Hawaii, Maui, Molokai, Oahu, Kauai—with four smaller islands close by—Kahoolawe, Lanai, Kaula, and Niihau. These make up the southeastern part of the archipelago; a number of smaller islands extend in a line to the northwest.

Hawaii itself, the most easterly and largest of the islands (4,021 sq mi / 10,414 sq km), is the top of a truly enormous undersea mountain. It has four volcanic peaks, two more than 13,000 ft (3,962 m) in altitude. Cooled lava covers a significant part of the island, due mostly to the frequent eruptions of Mauna Loa. Red Hill on Maui reaches 10,023 ft (3,055 m), but otherwise elevations in the archipelago are not nearly as high as on Hawaii, although all of the islands are hilly and home to volcanoes, especially toward the east. The coastlines are mostly rocky and rough. At the exposed northeastern shores erosion has produced sea cliffs 3,000 ft (914 m) high. Only Oahu and Niihau have large coastal plains. There is only one harbor, Pearl Harbor, west of Honolulu (the state capital and principal city) on Oahu.

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. Volcanic in origin, they form an arc consisting of more than 75 volcanoes extending 1,500 mi (2,414 km) from Mount Spurr opposite Cook Inlet, to Buldir Volcano between the Rat Islands and the Near Islands. Kodiak Island and St. Lawrence Island are another two large islands near Alaska.

Island Dependencies

Except for Puerto Rico (see entry on Puerto Rico), U.S. territories and dependencies consist of very small islands. These include American Samoa, Guam, the U.S. Virgin Islands, Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Islands, Navassa Island, Palmyra Atoll, and Wake Island.

The U.S. Virgin Islands comprises over 50 islands about 40 mi (64 km) east of Puerto Rico along the Anegada Passage in the Caribbean Sea, with a total land area of roughly 136 sq mi (352 sq km). Only three of the islands are important in size: St. Croix, St. Thomas, and St. John. The terrain is mostly hilly to rugged and mountainous, with little level land. About three-fourths of St. John is a national park. Navassa is a small island, about 2 sq mi (5.2 sq km) in area, between Haiti and Jamaica in the Caribbean. It has a central limestone plateau ringed by vertical white cliffs, with a terrain that is mostly exposed rock and some grassland.

All other U.S. dependencies are located in the Pacific Ocean. Baker Island, Howland Island, and Jarvis Island are all small islands of the Line Islands group, located south of Honolulu in the North Pacific Ocean. All three are coral islands. Johnston Atoll comprises two small islands, Johnston Island and Sand Island, west-southwest of Honolulu in the North Pacific Ocean. Their total land area is about 1 sq mi (2.8 sq km). The islands are flat, with a maximum elevation of 4 meters. Kingman Reef is a small, flat coral island with an area of less than 1 sq mi (2.6 sq km), about 993 mi (1,600 km) south-southwest of Honolulu. Palmyra Atoll is a small island grouping the Central Pacific with a land area of about 5 sq mi (13 sq km) about 993 mi (1,600 km) south-southwest of Honolulu. It comprises about 50 small islets covered with dense vegetation.

The Midway Islands are a group of low, flat coral islands, the largest of which are Eastern Island and Sand Island, with a total land and water area of 2 sq mi (5.2 sq km), about 1,459 mi (2,350 km) west-northwest of Honolulu. They are the westernmost islands of the Hawaiian Islands, but are not a part of that state. Wake Island, 2,298 mi (3,700 km) west of Honolulu, is an atoll about 3 sq mi (7 sq km) in area, consisting of three small coral islands formed on top of an underwater volcano. The central lagoon is a former crater and the islands are part of the rim.

American Samoa is part of the Samoan Archipelago, found about 2,300 mi (3,700 km) southwest of Hawaii. It is made up of seven islands—Tutuila, Tau, Olosega, Ofu, Aunuu, Rose, and Swain's—with a total land area of 77 sq mi (199 sq km). Most of the islands are volcanic with rugged peaks and limited coastal plains; two are coral atolls.

Guam is the southernmost and largest of the Mariana Islands, about 1,347 mi (2,170 km) west of Honolulu. It has a total land area of 209 sq mi (541 sq km). The island is of volcanic origin and is surrounded by coral reefs.

The Coastline

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. This is the largest bay of the embayed section of coast, but other notable bays are found further north, including the New York and Delaware Bays. The complex New England coastline includes the long, bent "arm" of Cape Cod and a number of islands.

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. Coastal land in this region is generally low and swampy. The final segment of the eastern coast is the smooth, sandy, east 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 is smoother. The westernmost part of the Gulf coast, in the Corpus Christi area, is the site of the Laguna Madre, a distinctive inland waterway that has a nearly identical counterpart—called by the same name—just south of the U.S. border along the coast of Mexico. These narrow strips of water are two of only three coastal lagoons in the world that are hypersaline (i.e. saltier than the ocean).

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 is a deep inlet with numerous islands at the northwestern corner of the conterminous United States; it is connected to the open ocean by the Strait of Juan de Fuca. San Francisco Bay is an inlet near the middle of the coast where the Sacramento River exits into the sea. The Columbia River also has a broad estuary further north, near Cape Disappointment.

The coast of Alaska is deeply embayed to the west, southwest, and south. Indentations in the south include Prince William Sound. The Alaska Peninsula extends southwest into the ocean, with Bristol Bay to the north. Further north is the Seward Peninsula. Point Barrow on the Arctic Ocean coast is the northernmost point in the country.

The Shore

The coast of northern New England coast is rocky, while the Atlantic coast south of New England is a plain with extensive sandy beaches. In some stretches of coastline south of New York City, the beaches border marshland areas. 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.

Unlike the Atlantic coast, the Pacific Coast has plains in only about half a dozen places. Much of the coast is mountainous, with steep bluffs, elevated marine terraces, and fewer beaches than the Atlantic and Gulf coasts. 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 narrow coastal plains.

CLIMATE AND VEGETATION

Temperature

Although the conterminous United States lies within the temperate zone of the Northern Hemisphere, between the Tropic of Cancer and 50° north latitude, there are wide variations in climate, including extremes in temperature and violent weather disturbances. The influence of the Atlantic and Pacific Oceans is limited by the mountain systems that intervene between the coasts and the interior both on the east and west, and also by the fact that air masses move primarily from west to east across the country. Thus, the states along the eastern seaboard have a continental climate despite their proximity to the Atlantic Ocean, as do the states of the Midwest, across which the same air masses move. These parts of the country have hot, humid summers and cold, snowy winters, except for the southeastern region, where winter weather is tempered by latitude. The mean annual temperature in Miami, Florida, is 76°F (24°C), while that in Boston is 51°F (11°C).

In the country's vast central lowlands, the tendency toward sharp contrasts and sudden changes is augmented by the interplay between cold arctic air masses that blow down from Canada and warm air masses moving north from the Gulf of Mexico. This combination can bring sudden shifts in temperature, summer thunderstorms, and more violent disturbances, including tornadoes and blizzards. The areas closest to the center of the country have some of the most extreme weather, including frigid winter temperatures. The northern Great Plains has seen summer highs of 121°F (49°C) and winter lows of -60°F (-51°C).

In contrast to the continental climate experienced in much of the country, the west coast, with its proximity of the Pacific Ocean, has a maritime climate with warm summers and mild winters that are rainy and overcast in the northwest but can include clear weather in the southern parts of California. Seattle, in the northern part of the Pacific coast has average temperatures of 39°F (4°C) in January and 65°F (18°C) in July. Farther south along the coast, Los Angeles averages 56°F (13°C) in January and 69°F (21°C) in July. Some of the maritime influence from the Pacific reaches the plateau region between the coastal ranges and the Rockies, but there are greater contrasts in temperature and much less precipitation.

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 73°F (23°C) in January and 80°F (27°C) in July.

Rainfall

Average annual rainfall is more than 40 in (100 cm) in an area covering roughly the eastern 2/5ths of the country, extending furthest west in the south. The prairie and Great Plains states to the north and west are considerably drier, with average rainfall as low as 18 in (46 cm) per year, dropping to 10 in (25 cm) in the northern plains.

In the Rocky Mountains, precipitation varies according to altitude, with higher elevations receiving more rain. The deserts to the west of Rockies are the driest parts of the country, although other parts of the intermontane region receive considerably heavier rainfall. The unevenly distributed precipitation of the region ranges from annual averages of 3 in (8 cm) in Yuma, Arizona to 30 in (76 cm) in higher parts of Arizona and New Mexico, to as much as 60 in (152 cm) even farther north, in central Idaho and Washington state. Annual rainfall in the Pacific coastal area varies widely with latitude, from 1.78 in (4.52 cm) in Death Valley in the south, to more than 140 in (356 cm) in the Olympic Mountains of Washington state.

In Alaska, the southern arc of the Aleutian islands and the panhandle have a wet maritime climate, while the interior is, on the whole, quite dry, in spite of its snow. Hawaii is generally moderately rainy (28 in / 71cm annually), but with very heavy rainfall at higher elevations. Mt. Waialeale, on the island of Kauai, can receive as much as 460 in (1,168 cm) of precipitation per year.

Grasslands

Almost half the country consists of plains. The Atlantic coastal plain, with a width of 100 to 200 mi (161 to 322 km), extends along the Atlantic seaboard south of the Hudson River and wraps around the Appalachian Mountains to follow the Gulf coast. It makes up about 10 percent of the country's land area, and, particularly in the south, contains the most fertile land in the country.

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, where rainfall is lightest, the rolling prairies are known as the Great Plains.

The vast central plain extends northward toward Canada and southward to the coastal plain bordering the Gulf of Mexico. Other extensive plains occur in the structural basins of the western mountains, such as the broad Central Valley of the Sacramento and San Joaquin rivers in California, and the Wyoming Basin.

Both tall and short grasses are found in the Great Plains and the Intermontane Plateau area between the Rockies and the Pacific coast. Species include buffalo, bunch, needle, wheat, and mesquite.

Deserts

The Colorado Plateau and the Great Basin are both characterized by arid areas of bare rock, and sparse vegetation elsewhere. Included in this region is the Great Salt Lake Desert, Death Valley, and the Sonoran Desert system, which extends southward into Mexico and also includes the Mojave Desert in southern California. The true deserts of the southwest support only scrub and annuals that appear only intermittently, after rainfall. Brush and bunchgrass are found in semidesert areas.

Tundra

Vast areas of Alaska are permanently frozen, and the state has thousands of glaciers. The area bordering the Arctic Ocean is a nearly featureless and permanently frozen coastal plain. Further south the lowlands thaw at the surface during the summer, but permafrost remains further down.

Forests and Jungles

A band of evergreen forest extends along the northern border from the east coast all the way to the Great Plains, with species such as pine, spruce, hemlock, and balsam. Farther south, this gives way to a mixed evergreen and deciduous forest that includes oak, maple, beech, walnut, ash, linden, and sycamore. Cypress and white cedar grow in the swamp areas of the Atlantic coastal plain. The most spectacular forests are found on the Pacific coast, which is famous for its giant redwoods, Douglas firs, and giant sequoias. The Olympia Mountains are the site of a temperate rainforest.

Small trees including dogwood, hawthorn, and wild cherry grow in many parts of the country. Some common species of the estimated 20,000 species of native flowering plants are dandelions, wild rose, black-eyed Susan, columbine, aster, phlox, and forget-me-not. Bamboos and ferns dominate the tropical woodlands of Hawaii.

The Everglades in southern Florida, which cover an area 50 mi (80 km) wide and 100 mi (160 km) long, are the country's last sizable subtropical wilderness. Sawgrass covers much of the region, which is inundated by a sheet of water 6 in (15 cm) deep. Fern-draped hardwood trees, including mahogany and other species, are found on its hammocks (small islands). Willows, cypress, mangroves, and slash pines are among the other trees that grow in the region, which is known for its diverse wildlife, including 347 bird species.

HUMAN POPULATION

In 2001 the overall population density of the United States was estimated at 75 people per sq mi (29 per sq km). However, the population of the United States is unevenly distributed across the country. The major population concentrations are along the northeast Atlantic and the southwest Pacific coastal regions. The Mid-Atlantic coastal region between New York City and Washington, D.C., is the most densely populated of all. Lesser concentrations of people can be found in the Midwest between the Great Lakes, Mississippi, and Ohio Rivers; Florida; and the rapidly growing southwest. The population is lowest in the arid interior of the west, the northern Great Plains, and in Alaska. More than three-quarters of the U.S. population is urban. This includes those people living in the suburbs around major cities, the growth of which was one of the major U.S. population trends of the last half of the twentieth century. Major cities can be found scattered throughout most of the country.

NATURAL RESOURCES

The United States has an abundance of almost every natural resource. Oil and natural gas are found in the south and off shore in the Gulf of Mexico. There are also major deposits in Alaska. There are huge coal reserves in both the Appalachian and Rocky Mountains. Iron and copper can be found in both of these ranges, as well as in the northern Great Lakes region. Gold and silver are mined in the west and Alaska. Large amounts of timber are harvested from the nation's forests, particularly those in the west. The many large rivers are a source of abundant water and hydropower. Huge tracts of farmland make the United States one of the world's leading producers of food.

FURTHER READINGS

Brinkley, Douglas. The Majic Bus: An American Odyssey. New York: Anchor Books, 1994.

Horwitz, Tony. Confederates in the Attic: Dispatches from the Unfinished Civil War. New York: Pantheon Books, 1998.

McPhee, John A. Coming into the Country. New York: Farrar, Straus & Giroux, 1977.

——. Basin and Range. New York: Farrar, Straus & Giroux, 1981.

Sierra Club. http://www.sierraclub.org (accessed July 12, 2002).

U.S. Geological Survey. USGS. http://www.usgs.com (accessed July 12, 2002).

U.S. National Park Service. ParkNet. http://www.nps.gov (accessed July 12, 2002).

GEO-FACT

Founded in 1872, Yellowstone National Park was the first national park ever established anywhere in the world, and is one of the oldest nature reserves. It is 3,472 sq mi (8,987 sq km) in area, covering parts of the Yellowstone Plateau and the Northern Rockies. Yellowstone is situated above a geothermal hotspot, a place where the magma normally found deep within the Earth has made its way close to the surface. As a result the entire park and surrounding region is very seismically active. In prehistoric times the park was the site of incredibly powerful volcanic eruptions. An eruption 600,000 years ago left a crater 50 mi (32 km) long and 30 mi (19 km) wide. This volcano, since buried, is still technically active. The most famous attractions of the park, its hot springs and geysers, are a result of all of this volcanic activity. Yellowstone National Park is thought to contain roughly 10,000 hot springs and geysers, more than half of all such features on Earth.