An endorheic basin (from ancient Greek: ἔνδον, ῖnon, "within" and ῖεῖν, rheîn, " flow ") is a closed drainage basin that contains water and does not allow other bodies of water, such as rivers or oceans, to converge in lakes or swamps, permanent or seasonal, that are balanced by evaporation. Such a basin can also be referred to as a closed or terminal basin or as an internal drainage system.
Typically, water that has accumulated in a drainage basin eventually flows through rivers or streams on the Earth's surface or through underground diffusion through permeable rocks, eventually ending up in the oceans. However, in an endorheic basin, the rain (or other precipitation) that falls into it does not flow, but can only leave the drainage system by evaporation and infiltration. The bottom of such a basin is typically occupied by a salt lake or a salt pan.
Endorheic regions, in contrast to exorheic regions that flow into the ocean in geologically defined patterns, are closed hydrological systems. Its surface waters drain to the internal terminal points where the water evaporates or infiltrates the ground, not having access to discharge into the sea. Endorheic bodies of water include some of the largest lakes in the world, such as the Aral Sea (formerly) and the Caspian Sea, the largest salt water in the world.
Most endorheic basins are arid, although there are many notable exceptions, such as the Valley of Mexico, the Lake Tahoe region, and several regions of the Caspian Basin.
Endorheic basins can be massively and rapidly affected by climate change and excessive water withdrawal, for example for irrigation. An exorheic lake is naturally kept at an overflow level, so the flow of water in the lake can be many times more than is necessary to maintain its current size. In contrast, an endorheic basin does not have sufficient inflow that overflows the ocean, so any loss of water intake can immediately begin to shrink the lake. In the past century, many very large endorheic lakes have been reduced to small remnants of their former size, such as Lake Chad and Lake Urmia, or they have completely disappeared like Lake Tulare and Lake Fucino. The same effect was seen at the end of the Ice Age, in which many extremely large lakes in the Sahara and western United States disappeared or were drastically reduced, leaving behind remaining desert basins, salt flats, and salt lagoons.
