In geography, a basin is a low area of the Earth's surface enclosed by higher ground, shaped roughly like a bowl, into which water, sediment or both collect. The word covers four related things: drainage basins (the land a river drains), endorheic basins (closed hollows with no outlet to the sea), structural or sedimentary basins (places where the crust has sunk and filled with rock), and ocean basins (the deep floors between continents).
Those four meanings overlap. Death Valley, in the image above, is all three of the land types at once: it is a sunken block of crust (structural), it collects the runoff of the ranges around it (drainage), and none of that water ever reaches the ocean (endorheic). Keeping the meanings apart is the first step to reading any map that uses the word.
Why the Same Word Means Four Things
What the four have in common is the bowl: a low centre surrounded by higher rims. What differs is what the geographer is interested in.
- Drainage basin (also catchment or watershed): all the land whose rain and snowmelt flows to one river and its tributaries. Defined by water flow.
- Endorheic basin: a drainage basin whose water never reaches the sea, leaving only by evaporation or seepage. Defined by having no exit.
- Structural or sedimentary basin: an area where the crust has subsided and filled with layers of sediment, sometimes kilometres thick. Defined by geology, and it may not look like a hollow at all today.
- Ocean basin: the deep sea floor between the continental margins. Defined by oceanic crust.
Drainage Basins: Where Every Raindrop Ends Up
Every point on land belongs to some drainage basin. The boundary between two basins is a drainage divide, usually a ridge or mountain crest; rain falling on one side ends in one river, and rain an arm's length away ends in another. On the largest scale, continental divides split whole continents: in North America the Great Divide along the Rockies separates water bound for the Pacific from water bound for the Atlantic and the Gulf of Mexico.
The largest drainage basins on Earth are:
- Amazon: roughly 7 million km², spread over nine countries and territories, with Brazil holding the largest share. The Amazon carries about a fifth of all the river water reaching the world's oceans.
- Congo: about 4 million km², the second-largest, straddling the equator so that part of it is always in a rainy season.
- Nile: around 3.2–3.4 million km² across 11 countries, though much of it is desert that contributes little water.
- Mississippi–Missouri: around 3.2 million km², draining all or part of 31 US states and two Canadian provinces.
Drainage basins matter because they are the natural unit for managing water. Pollution, dams and withdrawals anywhere upstream affect everyone downstream in the same basin, which is why agreements such as the 1960 Indus Waters Treaty between India and Pakistan are organised around basins rather than borders. For more on how divides and catchments work, see our explainer on what a watershed is, and for the two biggest basins side by side, our Amazon Basin vs Congo Basin comparison.
Closed Basins That Never Reach the Sea
Roughly a fifth of the world's land surface drains internally. In an endorheic basin, rivers flow inward to a terminal lake, marsh or salt flat, and because evaporation removes only water, the dissolved salts they carry are left behind. That is why so many closed-basin lakes are salty.
- The Great Basin of the western United States covers around 500,000 km², mostly in Nevada and Utah. It is really dozens of small closed basins separated by north–south ranges. Its best-known terminal lake is the Great Salt Lake, remnant of the Ice Age Lake Bonneville; our Great Basin Desert guide covers the region.
- The Caspian Sea, at about 371,000 km², is the largest lake on Earth and sits in the largest endorheic basin, fed chiefly by the Volga.
- The Dead Sea, with a shoreline around 430 m below sea level, is the lowest land on Earth and one of its saltiest lakes.
- The Aral Sea shows how fragile these systems are. Once the fourth-largest lake in the world, it shrank to a small fraction of its size after Soviet irrigation schemes diverted the Amu Darya and Syr Darya from the 1960s onward.
- Lake Chad and Australia's Kati Thanda–Lake Eyre (about 15 m below sea level) fill and shrink dramatically with wet and dry years.
Death Valley belongs here too. Badwater Basin on its floor, at 86 m below sea level, is the lowest point in North America, and the valley's rare floods simply spread out and evaporate, leaving the white salt pan visible from space.
How the Ground Sinks: Structural and Sedimentary Basins
To a geologist, a basin is a place where the crust has gone down and stayed down long enough to fill with sediment. There are several ways that happens:
- Rift basins form where the crust is pulled apart and blocks drop between faults. The East African Rift lakes and Death Valley's pull-apart structure are examples.
- Foreland basins form in front of growing mountain ranges, whose weight bends the crust downward. The Indo-Gangetic Plain in front of the Himalayas and Italy's Po Valley in front of the Alps and Apennines are both foreland basins filling with debris from the mountains beside them.
- Intracratonic basins sag slowly in the middle of old, stable continents. The Michigan Basin is a near-circular example: its rock layers form a bullseye, so the Great Lakes region's oldest rocks outcrop at the rim and the youngest at the centre. The Paris Basin has the same concentric pattern, which is why rings of chalk and limestone escarpments surround Paris.
These basins matter economically because organic material buried in thick sediment slowly becomes oil, gas and coal. The Permian Basin of west Texas and New Mexico, the Western Canada Sedimentary Basin and the basins beneath the Persian Gulf hold much of the world's hydrocarbon reserves.
Basin or Valley? How the Two Differ
The closest cousin of a basin is a valley, and people often use the words interchangeably. They are not the same:
- Shape: a valley is long and narrow, with a clear downhill direction. A basin is broad and roughly as wide as it is long, sloping inward from several sides.
- Openness: a valley is open at its lower end so its river can leave. A basin may be closed altogether.
- Origin: valleys are carved, by rivers (V-shaped) or glaciers (U-shaped). Basins are usually created by the crust sinking, or by being enclosed by uplifted ranges, rather than by cutting.
- Scale: a drainage basin contains many valleys. The Amazon Basin holds thousands.
Names do not always follow the rule. Death Valley is geologically a basin, not a carved valley, and California's Central Valley is a structural trough. Treat place names as history, not definitions.
Ocean Basins and the Floors Between Continents
The largest basins on Earth are underwater. The Pacific, Atlantic, Indian, Southern and Arctic ocean basins are floored by dense oceanic crust that sits lower than the lighter continents, with average depths of around 3,700–4,000 m. The Pacific Basin alone covers about a third of the planet's surface and holds its deepest point, Challenger Deep in the Mariana Trench, at roughly 10,900 m.
An ocean basin has its own anatomy. From the coast outward comes the shallow continental shelf, then the steeper continental slope, then the gentle continental rise, and finally the abyssal plains, blankets of fine sediment so even that they are among the flattest surfaces on Earth. Mid-ocean ridges run through the middle of the basins like seams, and chains of seamounts and volcanic islands such as Hawaii rise from their floors.
Unlike land basins, ocean basins are temporary on geological timescales. The Atlantic is widening by a few centimetres a year along the Mid-Atlantic Ridge, while the Pacific is slowly shrinking as its floor sinks back into trenches around the Ring of Fire.
Living at the Bottom of the Bowl
Basins can be wonderful places to live and difficult ones. Their floors are often flat and fertile: China's Sichuan Basin, ringed by mountains and nicknamed the Red Basin for its sandstone soils, supports well over 100 million people around Chengdu and Chongqing. But the same enclosing rims trap air. On calm winter days, cold air settles on the basin floor under a lid of warmer air, a temperature inversion that holds smog in place. Los Angeles, Mexico City (built on the drained lakebed of the Basin of Mexico), Salt Lake City and the Po Valley all suffer from it, and Mexico City is also sinking as groundwater is pumped from the old lake sediments beneath it.
Whether you are looking at a river map, an oil field, a salt flat or the deep sea, the question to ask is the same: what is collecting here, and why can it not get out? Test your map-reading on real landforms with our geography games.