Drainage basin

A drainage basin or river basin, also referred to as a catchment area and sometimes a river system (among other terms), is a unit of land, bounded by points of highest elevation, which drains water, via a network of rivers, into the sea or terminal basin. The largest drainage basin on Earth is the Amazon Basin, with a total area of 6.1 million square kilometres.

Open and closed basins
Most drainage basins, especially those in wet climates, drain into the world ocean. These are termed exorheic basins as the surplus of water exits the catchment as in an open system.

Alternatively, some basins (particularly those in arid climates) function as closed systems where discharge into the sea does not occur, and instead water flows to a topographic low (typically a lake). In some areas of very high evaporation, there may be no lake present at all. These are termed endorheic basins. Some of the largest endorheic regions include the Great Basin (North America), Lake Chad (Africa), the Aral Sea (Eurasia), and the Okavango Basin (Africa).

Additionally, some drainage basins may appear to be endorheic insofar as there is no visible surface water connection to the sea or another river, but are in fact cryptorheic as they drain via groundwater or karst systems.