An endorheic basin (from the Greek: ἔνδον, éndon, "within" and ῥεῖν, rheîn, "to flow"; also terminal or closed basin) is a closed drainage basin that retains water and allows no outflow to other bodies of water such as rivers or oceans. Normally the water accruing in drainage basins flows out through surface rivers or by underground diffusion through permeable rock to the oceans. However, in an endorheic basin, rain (or other precipitation) that falls within it does not flow out but may only leave the drainage system by evaporation and seepage.

The bottom of such a basin is typically occupied by a salt lake or salt pan. Endorheic basins are also called internal drainage systems. Endorheic regions, in contrast to exorheic regions which flow to the ocean in geologically defined patterns, are closed hydrologic systems.

Their surface waters drain to inland terminal locations where the water evaporates or seeps into the ground, having no access to discharge into the sea. Endorheic water bodies include some of the largest lakes in the world, such as the Aral Sea and the Caspian Sea, the world’s largest saline body of water cut off from the ocean. Endorheic lakes are bodies of water that do not flow into the sea.

Most of the water falling on earth finds its way to the oceans through a network of rivers, lakes and wetlands or through evaporation. However, there is a class of water bodies that are located in closed or endorheic watersheds where the topography prevents their drainage to the oceans. These endorheic watersheds (containing water in rivers or lakes that form a balance of surface inflows, evaporation and seepage) are often called terminal lakes or sink lakes.

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