Geography of the United States

Location and Size
The United StatesThe United States covers the entire midsection of North America. Alaska, in northwest North America, and Hawaii, in the Pacific Ocean, are parts of the United States.

Until 1959, when the territories of Alaska and Hawaii became the 49th and 50th states, the 48 contiguous (adjoining) states and the District of Columbia were known as the continental United States. These states are bounded on the north by Canada, on the east by the Atlantic Ocean, on the south by the Gulf of Mexico and the Republic of Mexico, and on the west by the Pacific Ocean. Alaska is separated from the main part of the country by more than 500 miles (800 km) of Canadian territory. Hawaii, a chain of volcanic and coral islands, lies about 2,400 miles (3,900 km) southwest of California in the Pacific Ocean.

With the addition of Alaska and Hawaii, the United States replaced Brazil as the world's fourth largest country. The northernmost point of the United States is Point Barrow, Alaska, 1,300 miles (2,100 km) from the North Pole; the southernmost point is Kalae (South Cape), Hawaii Island, Hawaii, at about the same latitude as Mexico City. The point farthest east is West Quoddy Head, Maine. The westernmost point, Attu Island, Alaska, is in the Eastern Hemisphere south of Siberia. The geographic center of the United States, excluding Hawaii, is in Butte County, South Dakota, 14 miles (23 km) east of where the Wyoming, South Dakota, and Montana borders meet.

From Hawaii to eastern Maine measures 5,100 miles (8,200 km); from Point Barrow to Florida's southern tip, 4,300 miles (6,900 km). The greatest east-west distance in the 48 contiguous states is 2,800 miles (4,500 km); the greatest north-south, 1,650 miles (2,660 km). The border with Canada totals 5,527 miles (8,895 km), including 1,540 miles (2,478 km) in Alaska. The Mexican border is 2,013 miles (3,240 km) long.

The United States coastline totals 12,383 miles (19,929 km), of which Alaska has 6,640 miles (10,686 km) and Hawaii 750 miles (1,207 km).

Physical Geography
Land

The United States contains a wide range of landforms, from lofty mountains to flat, arid deserts and vast, grassy plains. There are old mountain systems, rounded by the action of erosion; and young mountain ranges, still tall and jagged. The northern part of the country bears the scars of huge glaciers that covered the land during the last Ice Age. In some areas, especially Alaska, glaciers are still active. There are also active and dormant volcanoes, vast plateaus and basins, deep canyons, gorges, and valleys. Some of the lakes and rivers of the United States are among the world's largest.

The following text, describing the major physical regions of the United States, deals primarily with the 48 contiguous states. The physical features of Alaska and Hawaii are discussed fully in separate articles.

The Atlantic and Gulf Coastal Plain extends along the Atlantic and Gulf coasts from Cape Cod, Massachusetts, to Mexico. The plain was formed millions of years ago when deep marine sediments were deposited on the ocean's floor. Eventually these sediments emerged to become dry land. This flat coastal lowland varies in width from only a few miles in New England to more than 500 miles (800 km) in the lower Mississippi River valley. Sandy beaches, bars, and islands border the coast; swamps and marshes are often found short distances inland.

The inner margin of the Atlantic Coastal Plain is marked in part by the fall line, where rivers descend from the Piedmont Plateau, forming rapids and waterfalls. Washington, D.C., Richmond, Virginia, Columbia, South Carolina, and many other southeastern cities are located on or near the fall line.

The Appalachian Highlands is a region of mountains, valleys, and plateaus extending southwestward from New England to Alabama. The Appalachians are one of the oldest mountain systems in the country. Through the ages they have been worn down relatively low. The region is divided into five sections—New England, the Piedmont Plateau, the Blue Ridge, the Ridge and Valley, and the Appalachian Plateau:

New England is largely an area of low, rounded mountains. Peaks rising to more than 4,000 feet (1,200 m) above sea level are found mainly in the Green Mountains of Vermont and the White Mountains of New Hampshire.

The Piedmont Plateau or Piedmont, stretching from the mouth of the Hudson River to Alabama, is a hilly plateau section bordering the Atlantic Coastal Plain.

The Blue Ridge section rises sharply from the Piedmont Plateau, from Pennsylvania to Georgia. This mountainous section includes the Great Smoky, Blue Ridge, and Black Mountains. Mount Mitchell, a 6,684-foot (2,037-m) peak in North Carolina's Black Mountains, is the highest point in the East.

The Ridge and Valley section west of the Blue Ridge section, is made up of many steep-sided ridges running from New York to Alabama. Except for the broad Great Valley, which continues on into Canada, the valleys separating the ridges are short and narrow.

The Appalachian Plateau in the westernmost part of the highland region, is cut by many deep river valleys and has a mountainous appearance. Parts of this plateau include the Catskill Mountains in New York; the Allegheny Plateau, largely in Pennsylvania and West Virginia; and the Cumberland Plateau, mainly in Kentucky and Tennessee.

The Interior Plains, the largest physical region in the United States, are located west of the Appalachian Highlands, east of the Rocky Mountains, and north of the Gulf Coastal Plain. The region's greatest dimensions are about 1,200 miles (1,900 km) east-west and 1,300 miles (2,100 km) north-south. Thick beds of sedimentary rocks underlie this vast plains area. North of the Missouri and Ohio rivers are deep glacial deposits.

At about the 100th meridian, the Interior Plains divide into the Great Plains, to the west, and the Central Lowlands, to the east. Both sections have vast stretches of flat land and of gently rolling land. A major difference between the two areas is elevation—the Great Plains lie at an elevation of 2,000 to 6,000 feet (600 to 1,800 m), the Central Lowlands are much lower.

The Interior Plains are bordered by two upland regions—the Superior Uplands on the north, and the Central Uplands on the south.

The Superior Uplands are in Minnesota, Wisconsin, and, Michigan. They are part of the huge Canadian Shield and rise to a maximum height of about 2,000 feet (600 m).

The Central Uplands are in Missouri, Arkansas, and Oklahoma. They include the Ozark Plateau and the Boston and the Ouachita Mountains. The Ozarks rise to about 2,500 feet (760 m); the Ouachitas, to 3,000 feet (900 m).

The Rocky Mountains rise abruptly from the Great Plains and extend northwesterly from northern New Mexico to Canada. Compared with the Appalachians, the Rockies are geologically young and are considerably more rugged. Within the Rockies are scores of ranges, many of which are separated by broad valleys and basins. The Wyoming Basin, in Wyoming, Colorado and Utah, is the largest of the mountain basins and divides the Rocky Mountains into northern and southern branches.

Among the many high ranges in the Southern Rockies are the Front, Park, and Sawatch ranges, in Colorado. Those in the Northern Rockies include the Bighorn Mountains and the Teton Range, in Wyoming; the Uinta Mountains and the Wasatch Range, in Utah; the Lost River Range, in Idaho; and the Bitterroot Range, on the Idaho-Montana border. On the whole, the Northern Rockies decline in height toward the north, while the Southern Rockies become lower toward the south.

The highest of the Rocky Mountain peaks are Mounts Elbert, Massive, and Harvard, in Colorado—all extend more than 14,400 feet (4,390 m) above sea level. Although not as high, Colorado's Pikes Peak (14,110 feet [4,301 m]) is more famous.

The Pacific Coast extends from Canada to Mexico along the Pacific Ocean and is composed chiefly of high mountain ranges and intervening fertile valleys. Bordering the coast, from northern Washington to southern California, are the Coast Ranges. They include Washington's Olympic Mountains, Oregon's Coast Range, the Klamath Mountains on the Oregon-California border, and numerous low ranges in California. Altitudes vary from about 2,000 feet (600 m) in the Coast Range to 9,000 feet (2,700 m) in the Klamath Mountains. In many areas the rugged coast is marked by steep cliffs and rocky headlands.

East of the Coast Ranges are the Cascade Range of Washington and Oregon, and the high Sierra Nevada of California. Lofty volcanic peaks dot the Cascades, towering high above the other mountains. Mount Rainier, the highest of these old volcanoes, rises 14,410 feet (4,392 m) above sea level. The rugged Sierra Nevada range includes Mount Whitney, which rises 14,494 feet (4,418 m) and is the highest peak in the United States outside of Alaska (Mount McKinley, in the Alaska Range, rises to 20,320 feet [6,194 m]). In Death Valley, less than 100 miles (160 km) southwest of Mount Whitney, is the lowest point in the country—282 feet (86 m) below sea level.

Within the Pacific Coast region are two large lowlands, almost completely enclosed by mountains. One is the Puget Sound-Willamette Valley, in western Washington and Oregon; the other, the Central Valley of California.

The Intermountain Region, lying between the Rocky Mountains and the Pacific Coast region, is a vast semiarid and arid land. Among its many and colorful landforms are mountains, plateaus, buttes, mesas, gorges and canyons, salt lakes, and deserts. There are three main sections of the Intermountain region—the Colorado Plateau, the Basin and Range section, and the Columbia Plateau:

The Colorado Plateau is in the southeast. It covers parts of Colorado, Utah, Arizona, and New Mexico, and varies in elevation from 5,000 to 11,000 feet (1,500 to 3,350 m). Many deep gorges cut the plateau, the Grand Canyon of the Colorado River being the largest and deepest.

The Basin and Range section, covering parts of Utah, Nevada, California, Arizona, New Mexico, and Texas, is a vast dry area of low mountains, most of them running north and south, and broad basins. Some of the ranges attain heights of 10,000 feet (3,000 m), but most are lower. The northern part of the Basin and Range, mainly in western Utah and Nevada, is occupied by the Great Basin. It is completely surrounded by higher land, and the waters that drain into it either evaporate or flow into salt lakes.

The Columbia Plateau in parts of Washington, Oregon, and Idaho, is a flat expanse of thick volcanic lava. Here the Columbia and Snake rivers have cut deep canyons and gorges.

Rivers

The Continental Divide, in most areas, follows the crest of the Rocky Mountains. Rivers east of the divide drain into the Atlantic Ocean, the Gulf of Mexico, or Hudson Bay. Those to the west drain into the Gulf of California, the Pacific Ocean, or the Great Basin, which has no outlet to the sea.

The Atlantic Ocean receives the water of the Great Lakes, the St. Lawrence River, and the many rivers east of the Appalachian Highlands. The St. Lawrence River is the largest river on the east coast, but most of its course is in Canada. Other eastern rivers are much shorter than the St. Lawrence but are nevertheless of great value, especially for purposes of navigation and for the harbors formed by their mouths. These rivers include the Penobscot, Kennebec, Merrimack, Connecticut, Hudson, Delaware, Potomac, James, Roanoke, Santee, Savannah, and Altamaha.

Most of the rivers between the Appalachian and Rocky Mountains are part of the Mississippi River system, which drains into the Gulf of Mexico. Chief tributaries of the Mississippi include the Ohio, Missouri, Arkansas, and Red rivers. Other rivers in the Mississippi basin are the Wisconsin, Illinois, Wabash, Allegheny, Monongahela, Kanawha, Kentucky, Cumberland, and Tennessee rivers, to the east of the Mississippi River; and the Yellowstone, Cheyenne, Platte, Cimarron, and Canadian rivers, to the west.

River improvements in the Mississippi basin have been chiefly for navigation, flood control, recreation, and conservation. On some rivers, however, such as the Tennessee, Cumberland, and Missouri, much water power has been developed.

Except for tributaries of the Mississippi, rivers on the Gulf Coastal Plain flow directly into the Gulf of Mexico. From east to west, these rivers include the Apalachicola-Chattahoochee, Mobile-Alabama-Coosa, Tombigbee, Pearl, Sabine, Trinity, Brazos, and Rio Grande.

The Red River of the North and its tributaries in Minnesota and North Dakota are the most important streams that flow northward to Hudson Bay.

West of the Rockies, many rivers descend from the high mountains and plateaus. There are, however, few large streams except for the Yukon, Kuskokwim, Columbia, Snake, and Clark Fork rivers, in the north; the Colorado, Green, and Gila rivers, in the south; and the Sacramento and San Joaquin rivers, in the Central Valley of California. Of these, the Colorado flows into the Gulf of California, and the rest empty into the Pacific Ocean. Rivers flowing into the Great Basin have no outlet to the sea; the Humboldt River, in Nevada, is the largest of these. Western rivers are used widely for irrigation and water power.

Lakes

The Great Lakes—Superior, Huron, Michigan, Erie, and Ontario—are the largest lakes of the United States. Of the five, only Lake Michigan lies entirely within the United States. The rest are shared with Canada. A total of 60,440 square miles (156,540 km2) of the Great Lakes is part of the United States.

Smaller lakes abound in several sections. In the North, the land from Minnesota to New England is dotted with thousands of lakes that were formed during the time glaciers covered the land. The largest of these is the Lake of the Woods, in Minnesota and Canada.

In the dry regions of the West are many salt lakes, of which the Great Salt Lake, in Utah, is the largest. Smaller salt lakes include the Salton Sea, in California, and Pyramid Lake, in Nevada. High in the western mountains are some of the nation's most scenic lakes. Among them are Lake Tahoe, in California and Nevada; Crater Lake, in Oregon; and Yellowstone and Jackson lakes, in Wyoming.

Of the many lakes and lagoons in the Gulf Coast area, Lake Pontchartrain, in Louisiana, and Lake Okeechobee, in Florida, are the largest.

Climate

The climates of the United States vary greatly from section to section. Temperatures have reached such extremes as — 80° F. (-62° C.) in Alaska and 134° F. (57° C.) in California. Annual rainfall also varies widely—from a record high of 460 inches (11,680 mm) in central Kauai, Hawaii, to an annual average of less than 2 inches (50 mm) in the deserts of the Southwest.

On the whole, the interior of the country experiences the greatest seasonal change in temperature. It heats quickly in summer and cools rapidly in winter. In some coastal areas, because of the influence of offshore ocean currents, there is relatively little change throughout the year.

Except for Alaska and Hawaii, the United States is situated in the middle latitudes—the so-called temperate zone. Here, cold polar air masses from the north meet warm subtropical air masses from the south. The meeting of these masses of air produces cyclonic storms, which are carried eastward across the country by the prevailing westerly winds. Most of the nation's rain and snow occurs in conjunction with cyclonic storms.

The following broad types of climate are found:

Humid Continental

ranging from the Dakotas, Nebraska, Kansas, and Oklahoma eastward to the Atlantic Ocean. Within this region, there are two phases of the climate—a northern and a southern phase. The northern phase occurs in the northernmost states from the Dakotas to Maine. Summers are short and cool; winters are long, snowy, and severely cold. Temperatures in Duluth, Minnesota, for example, average 39° F. (4° C.) annually, 9° F. (-13° C.) in January, and 66° F. (19° C.) in July. Annual precipitation averages 30 inches (760 mm); snowfall, 78 inches (1,980 mm). (Precipitation figures include melted snow and ice pellets.)

The southern phase is characterized by long, hot summers and short, cold winters. For example, temperatures in St. Louis, Missouri, average 56° F. (13° C.) annually, 31° F. (-1° C.) in January, and 79° F. (26° C.) in July. Yearly precipitation averages 36 inches (910 mm); snowfall, 19 inches (480 mm).

Humid Subtropical

extending from eastern Texas to the Atlantic Coast. As a whole, the region receives the largest amount of rainfall in the country, from 40 to 60 inches (1,000 to 1,500 mm) annually. Summers are long and hot; winters are short and mild. In Jackson, Mississippi, for example, temperatures average 65° F. (18° C.) annually, 47° F. (8° C.) in January, and 82° F. (28° C.) in July. Average yearly rainfall is 49 inches (1,240 mm); snowfall, about 1 inch (25 mm).

Humid Tropical

found on the southern tip of Florida and in Hawaii. Warm weather prevails throughout the year; rainfall is abundant; and snow is unknown. Temperatures in Miami, for example, average 75° F. (24° C.) annually, 67° F. (19° C.) in January, and 82° F. (28° C.) in July. There is normally about 60 inches (1,520 mm) of rain yearly. Honolulu has somewhat warmer winters than Miami, milder summers, and less rainfall.

Mediterranean

found in central and southern California. As implied by the name, this type of climate is similar to that of regions bordering the Mediterranean Sea. The climate is marked by scant rainfall; long, hot, dry summers; and short, mild, rainy winters. Snow and freezing weather are infrequent. For example, temperatures in downtown Los Angeles average 65° F. (18° C.) annually, 57° F. (13° C.) in January and 73° F. (23° C.) in July. Annual rainfall is about 14 inches (360 mm).

West Coast Marine

extending along the Pacific Coast from northern California to the Canadian border. Because of the protection provided by the Cascade Range and the tempering influence of the Pacific Ocean, the climate varies little throughout the year. Winters are mild and summers are cool. In Seattle, Washington, for example, temperatures average 52° F. (11° C.) annually, 40° F. (4° C.) in January and 66° F. (19° C.) in July. Annual precipitation averages about 36 inches (910 mm); snowfall, about 8 inches (200 mm). A slightly cooler version of this climate, with considerably more rainfall, is found along Alaska's southeastern coast.

Middle Latitude Steppe and Desert

predominating over the Great Plains and most of the Intermountain region. Rainfall, scant and unreliable throughout this zone, decreases toward the southwest, where deserts are found in parts of Nevada, Utah, California, and Arizona. Throughout the region, summers can be very hot and winters extremely cold. Winters in the northern section are particularly long, cold, and snowy. In Great Falls, Montana, for example, temperatures average 45° F. (7° C.) annually, 21° F. (-6° C.) in January, and 69° F. (21° C.) in July. Annual precipitation averages about 15 inches (380 mm); snowfall, about 58 inches (1,470 mm).

Tropical Desert

found in southwestern Arizona and southeastern California. The climate is marked by extreme dryness; an abundance of sunshine; long, hot summers; and warm winters. For example, Phoenix, Arizona, averages 70° F. (21° C.) annually, 91° F. (33° C.) in July, and 51° F. (11° C.) in January. Annual rainfall averages about 7 inches (180 mm).

Subarctic

found in the interior of Alaska and along the Bering Sea and Arctic Ocean coasts. Winters are severely cold, averaging -12° F. (-24° C.) in Fairbanks and -16° F. (-27° C.) in Barrow in January. July temperatures average 61° F. (16° C.) at Fairbanks and 39° F. (4° C.) at Barrow. There is considerable snow, but limited annual precipitation in general.