Geography of the United States: a lowland basin between two mountain systems

Why sources disagree on the country's size

The fifty states and the federal district have been measured many times, and the figure keeps moving. From 1989 to 1996 the official total was 9,372,610 square kilometres of land and inland water. The Great Lakes and coastal waters were added in 1997, bringing it to 9,629,091, and territorial waters in 2007, bringing it to 9,826,630. By 2026 the published totals differed by more than 300,000 square kilometres between sources.

Within that total the states differ enormously. Britannica gives Rhode Island 4,001 square kilometres and Alaska 1,723,337, so that the one would fit into the other about four hundred times. Forty-eight states lie together between Canada and Mexico, the contiguous United States or the Lower 48. Alaska sits at the northwestern corner of the continent and Hawaii in the middle of the Pacific. Five inhabited territories, Puerto Rico and the Virgin Islands in the Caribbean and Guam, the Northern Mariana Islands and American Samoa in the Pacific, lie outside those totals.

The land borders run 8,891 kilometres with Canada and 3,145 kilometres with Mexico. NOAA measures the shoreline of the fifty states at 151,475 kilometres.

What lies between the two mountain systems

Britannica describes the centre of the contiguous country as a sprawling interior lowland, reaching from the ancient shield of central Canada to the Gulf of Mexico. Its base is a platform of crystalline rock that has not been folded by mountain building for more than 600 million years. In the United States that platform is mostly buried under sedimentary rock, and it comes to the surface only in the far north: in the Adirondack Mountains of New York and the Superior Upland of Minnesota, Wisconsin and Michigan, where the iron of the Mesabi Range lies close to the ports of the upper Great Lakes.

South of that edge begins the Central Lowland, which runs about 2,400 kilometres from New York to central Texas. Heights above 600 metres are unusual there. The northern half was shaped by ice sheets that left a mosaic of boulders, gravel, clay and lakes; the southern half is blanketed in loess, silt laid down by the wind. The Ozark Plateau and the Ouachita Mountains of Missouri, Arkansas and Oklahoma form the one hilly interruption between the Appalachians and the Rockies.

West of the Mississippi the Great Plains run the full distance from Canada to Mexico in a band about 800 kilometres wide, built of sand, silt and gravel washed east from the Rockies. Their surface climbs from about 600 metres near Omaha to more than 1,825 metres at Cheyenne, so gradually that popular belief holds them flat. Before farming took them over they were grassland, tallgrass prairie in the east and shortgrass steppe on the High Plains.

Why the Appalachians are low and old

The Appalachians run nearly 2,400 kilometres from northeastern Alabama to the Canadian border. They are the eroded stumps of much higher ranges, folded so long ago, probably in the Permian, that weathering has left only the resistant rocks standing. Their subdivisions lie in bands parallel to the Atlantic. The Blue Ridge carries the highest summit, Mount Mitchell in North Carolina at 2,037 metres, and falls on its seaward side to the rolling Piedmont, the most productive farmland in the country before the Midwest was settled.

West of the Blue Ridge, folded sedimentary rock has been cut into the Ridge and Valley country, long ridges about 300 metres high running for tens of kilometres beside broad valleys. The Great Valley, which runs nearly the whole length of the chain, has been one of the main routes through it since pioneer times. Beyond lies the Appalachian Plateau, whose thick seams of bituminous coal fed the steel mills along the lower Great Lakes. New England, scoured by ice, has thin soils and the White Mountains, where Mount Washington reaches 1,917 metres.

How the western Cordillera was built

The Cordillera covers fully a third of the United States and belongs to the chain of mountains that rings the Pacific. Its eastern wall is the Rocky Mountains, a discontinuous series of ranges from New Mexico to Canada whose highest summit, Mount Elbert in Colorado, reaches 4,400 metres. The Rockies rise abruptly from the Great Plains.

Between the Rockies and the Pacific ranges lies a region Britannica treats as separate from the mountains around it, the Intermontane Plateaus. The Great Basin is a desert of salt flats, closed basins and short north-south ranges. The Colorado Plateau, centred on the Four Corners, holds the Grand Canyon, Arches, Mesa Verde and Bryce Canyon. The Columbia Plateau covers eastern Washington, western Idaho and northeastern Oregon.

The Pacific ranges close the country off from the sea without a coastal plain. The Cascades are a line of volcanoes; Mount St. Helens in Washington erupted in 1980. The Sierra Nevada carries Mount Whitney at 4,421 metres, 136.2 kilometres from Badwater Basin in Death Valley, 85 metres below sea level. West of them lie the Central Valley of California and the Willamette Valley of Oregon. The whole coast sits on the Pacific Ring of Fire, and California and southern Alaska have suffered destructive earthquakes, in San Francisco in 1906 and near Anchorage in 1964.

Where the Mississippi carries the water

The Mississippi drains much of the land between the Appalachians and the Rockies. Its length depends on the measurer and the year, because its delta grows and erodes: the Geological Survey has published 2,300 miles, the Environmental Protection Agency 2,320 and the National Park Service 2,350, from Lake Itasca in Minnesota to the Gulf. Its tributary the Missouri is about 100 miles longer. Water leaving Lake Itasca takes about three months to reach the sea, and the river discharges 16,792 cubic metres a second at its mouth.

It is also a road. Barges move about 175 million tons of freight a year on the upper Mississippi through 29 locks and dams, and its two large eastern tributaries, the Ohio and the Tennessee, drain the country west of the Appalachians.

Four of the five Great Lakes form part of the Canadian border; only Lake Michigan lies wholly inside the United States. In the West the Columbia and the Colorado rise far inland near the eastern edge of the Cordillera and flow through plateaus and basins to the sea.

How the Atlantic coast differs from the Pacific

The Atlantic and Gulf coasts are low. A coastal plain runs from Cape Cod to beyond the Mexican border and continues under the sea as a continental shelf reaching up to 400 kilometres offshore. Florida is an upwarp of that plain, and nowhere in its 640-kilometre length does it rise more than 100 metres above sea level. North of New York the weight of the ice sheets pushed the plain under the sea, and the Atlantic breaks directly on New England's rock; Cape Cod and Long Island are what remains of it.

South of New York the sea has flooded the lower valleys of the coastal rivers. Chesapeake Bay is the drowned valley of the Susquehanna. A line of barrier beaches runs for more than 1,600 kilometres along the coast. The Gulf Coast of Texas and Louisiana holds oil and gas in domed structures, both on land and offshore, and its soils are mostly poor, with exceptions such as central Florida's citrus country and the old Cotton Belt.

The Pacific coast is hilly or mountainous almost everywhere. It was shaped by recent crustal movement, and it still has more relief than the Atlantic.

What lies beyond the contiguous states

Alaska, bought from Russia in 1867, is a separate world from the Lower 48. Tall mountain ranges rise sharply from broad, flat tundra plains, and the chain of islands off its south and southwest coasts is lined with volcanoes. Denali, at 6,190 metres, is the highest summit in the country. In the north, tundra and Arctic conditions prevail.

Hawaii lies far to the southwest in the middle of the Pacific, a chain of tropical volcanic islands that belongs physically and culturally to Polynesia. Several of its volcanoes are active, Kilauea among them, although no eruption there has threatened life on a large scale since the seventeenth century. The islands are also among the wettest ground in the country: the Big Bog on Maui averages 10,260 millimetres of rain a year.

The inhabited territories add two more oceans. Puerto Rico and the Virgin Islands, which the United States bought from Denmark, are tropical islands in the northeastern Caribbean, exposed to the same Atlantic hurricanes as Florida and the Gulf Coast. Guam and the Northern Mariana Islands occupy the limestone and volcanic islands of the Mariana archipelago in the western Pacific. American Samoa, a group of volcanic peaks and coral atolls in the Samoan chain, is the only populated territory south of the equator.

What the climate does from Alaska to Florida

The country spans almost every climate there is: subtropical in the South, continental in the north, tropical in Hawaii and southern Florida, polar in Alaska, semiarid on the Great Plains west of the 100th meridian, Mediterranean on the California coast and arid in the Great Basin and the Southwest. The polar jet stream moves into Canada in summer and back over the United States in winter, bringing Pacific storms ashore in the Northwest. The Cascades, the Sierra Nevada and the Rockies wring most of the moisture out of them.

The contrasts are wide. The Quinault Rainforest in Washington averages 3,480 millimetres of rain a year; Yuma in Arizona averages 67. The Mount Baker Ski Area reported 2,896 centimetres of snow in the winter of 1998 to 1999. The Great Basin and Columbia Plateau lie in the rain shadow of the Pacific ranges and receive less than 380 millimetres, and the Southwest can stay above 37.8 °C for weeks in summer. Northern Alaska has fallen to minus 62.2 °C. At the other end, Florida is subtropical in the north and tropical in the south, with wet summers and dry winters, and much of it and the Deep South is frost-free, which lets a large citrus industry grow in the centre of the state. South and east of the Great Lakes, lake-effect snow piles up in bands: the Buffalo area received more than 1.52 metres over the winter of 2006 to 2007.

The open interior brings violent weather. Cold air from the north and moist air from the Gulf of Mexico meet over the Great Plains, and around 1,000 tornadoes touch down each year, most of them in the strip from north Texas to Nebraska and east into Tennessee called Tornado Alley. The hurricane season on the Atlantic and Gulf coasts runs from 1 June to 30 November. Drought is rare but has been catastrophic: the Dust Bowl of 1931 to 1942 emptied whole regions of the southern plains. The Census Bureau counted 3.3 million people displaced by natural disasters in 2022, half of them by hurricanes.