exorheic

Exorheic describes a drainage system in which water ultimately flows out of the land surface to the sea or ocean. In contrast to endorheic basins, where water collects in inland lakes and evaporates without reaching the coast, an exorheic drainage system has a surface outlet that connects rivers to the coast.

The term is used in hydrology to categorize basins by their hydrological endpoint. Exorheic basins include most of the world’s major river systems, whose waters reach the oceans through rivers, deltas, and coastal outlets, while endorheic basins lack a surface outlet to the sea and lose water mainly by evaporation or seepage.

Most major river basins worldwide are exorheic. In arid or mountainous regions, drainage patterns can be intermittent or highly seasonal, but the presence of a defined outlet to the sea characterizes an exorheic system. Endorheic regions include areas such as the Caspian Sea region and the Great Basin of North America, as well as parts of Central Asia and northwestern China, where surface drainage does not reach the ocean.

The exorheic or endorheic status of a basin can change over geological timescales due to tectonic uplift, river capture, climate change, and sedimentation, which can open or close outlets and alter drainage patterns. Understanding exorheic drainage helps in studies of regional hydrology, climate interactions, and paleoenvironmental reconstruction.