A basin is a depression, or dip, in the Earth’s surface. Basins are shaped like bowls, with sides higher than the bottom. They can be oval or circular in shape, similar to a sink or tub you might have in your own bathroom. Some are filled with water. Others are empty.

Basins are formed by forces above the ground (like erosion) or below the ground (like earthquakes). They can be created over thousands of years or almost overnight.

The major types of basins are river drainage basins, structural basins, and ocean basins.

River Drainage Basins

A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds.

A watershed is a small version of a river basin. Every stream and tributary has its own watershed, which drains to a larger stream or wetland. These streams, ponds, wetlands, and lakes are part of a river basin. The Mississippi River basin in the U.S., for instance, is made up of six major watersheds: the Missouri, Upper Mississippi, Ohio, Tennessee, Lower Mississippi, and Arkansas-Red-White Rivers.

Every river is part of a network of watersheds that make up a river system’s entire drainage basin. All the water in the drainage basin flows downhill toward bigger rivers. The Pease River, in northern Texas, is part of the Arkansas-Red-White watershed. It is a tributary of the Red River. The Red River is a major tributary of the Mississippi River, which flows into the Gulf of Mexico.

The Amazon Basin, in northern South America, is the largest in the world. The Amazon River and all of its tributaries drain an area more than 7 million square kilometers (about 3 million square miles).

Structural Basins

Structural basins are formed by tectonic activity. Tectonic activity is the movement of large pieces of the Earth’s crust, called tectonic plates. Tectonic activity is responsible for such phenomena as earthquakes and volcanoes. The natural processes of weathering and erosion also contribute to forming structural basins.

Structural basins form as tectonic plates shift. Rocks and other material on the floor of the basin are forced downward, while material on the sides of the basin are pushed up. This process happens over thousands of years. If a basin is shaped like a bowl, a structural basin is shaped like a series of smaller bowls, stacked inside each other. Structural basins are usually found in dry regions.

Some structural basins are known as endorheic basins. Endorheic basins have internal drainage systems. This means they don’t have enough water to drain to a stream, lake, or ocean. The water that trickles into these types of basins evaporates or seeps into the ground.

When enough water collects in an endorheic basin, it can form a very salty lake, such as the Dead Sea, between Israel and Jordan. While water evaporates into the atmosphere, minerals remain. The remaining water becomes even saltier. The Dead Sea is one of the saltiest natural body of water on Earth. Its shore, about 400 meters (1,300 feet) below sea level, is Earth’s lowest dry point.

Death Valley, in the U.S. state of California, is another endorheic basin. At about 86 meters (282 feet) below sea level, it is the lowest place in North America. The water draining into Death Valley from its few streams does not exit the basin to a river or estuary. It evaporates or seeps into the ground.

A lake basin is another type of structural basin. Lake basins often form in valleys blocked by rocks or other debris left by a landslide, lava flow, or glacier. The debris acts as a dam, trapping water and forming a lake. Hunza Lake in Pakistan was formed when an earthquake triggered a massive landslide in 2010. The debris dammed the Hunza River, in addition to killing 20 people and destroying the village of Attabad. The Hunza River continues to flow into the lake basin, and many geologists and villagers worry the basin won’t be strong enough to hold the water.

Lake basins may also be carved out by glaciers—huge masses of ice—as they move down valleys or across the land. When the glaciers move, the basins they create remain. During the last ice age, glaciers carved the basins of the Finger Lakes, in the U.S. state of New York.

Sedimentary basins are a type of structural basin that aren’t shaped like typical basins, sometimes forming long troughs. Sedimentary basins have been filled with layers of rock and organic material over millions of years. Material that fills up the basin is called sediment fill.

Sedimentary basins are key sources of petroleum and other fossil fuels. Millions of years ago, tiny sea creatures called diatoms lived and died in ocean basins. Eventually, these ancient oceans dried up, leaving dry basins. The remains of the diatoms were at the bottom of these basins. The remains were crushed under billions of tons of sediment fill, over millions of years. In the right conditions, the pressure of the sediment fill turns the diatom remains into petroleum.

The Niger Delta sedimentary basin, in the countries of Nigeria, Cameroon, and Equatorial Guinea, is one of the most productive petroleum fields in Africa. In North America, the Western Canadian Sedimentary Basin is one of the continent's largest suppliers of gas and coal.

Ocean Basins

Ocean basins are the largest depressions on Earth. Edges of the continents, called continental shelves, form the sides of ocean basins.

There are five major ocean basins, coordinating with the major oceans of the world: the Pacific basin, the Atlantic basin, the Indian basin, the Arctic basin, and the Southern basin. Many smaller basins are often considered oceanic basins, such as the North Aleutian Basin, between the Pacific and Arctic Oceans.

Tectonic activity constantly changes ocean basins. Seafloor spreading and subduction are the most important types of tectonic activity that shape ocean basins.

Seafloor spreading happens along the boundaries of tectonic plates that are moving apart from each other. These areas are called mid-ocean ridges. New seafloor is created at the bottom, or rift, of a mid-ocean ridge. Ocean basins that have mid-ocean ridges are expanding. The Atlantic basin, for instance, is expanding because of seafloor spreading.

Subduction happens along the boundaries of tectonic plates that are crashing into each other. In these subduction zones, the heavier plate moves underneath, or subducts, the lighter one. Ocean basins that experience subduction, such as the Pacific basin, are shrinking.

Even though ocean basins make up more than 70 percent of the total land on Earth, scientists know relatively little about them. Some oceanographers (and some astronomers!) say that we know more about the surface of the moon than we do about the surface of the ocean floor.

It is very difficult to get information about landforms of the ocean basin, such as trenches and mid-ocean ridges. These landforms are thousands of feet below the surface of the water. Few instruments can endure the intense pressure, cold, and dark at the bottom of ocean basins. Occasionally, researchers themselves explore ocean basins in special submarines called submersibles.