An endorheic basin, from the Ancient Greek endon, meaning “within” and rhein, meaning “to flow”, is a closed drainage basin that holds onto water and does not allow outflow to other external bodies of water, such as oceans or rivers, but joins instead into lakes or swamps, seasonal or permanent, that equilibrate via evaporation. Basins such as these might also be referred to as closed or terminal basin or as internal drainage systems.
Usually, water that has built up in a drainage basin inevitably flows out through streams or rivers on the surface of the Earth or via underground diffusion through permeable rock, eventually ending up in the oceans. However, in an endorheic basin, rain or other precipitation that falls within it doesn’t flow out but may leave the drainage system only by seepage or evaporation. The bottom of such a basin is usually in the form of a salt lake or salt pan.
Endorheic areas, compared to exorheic areas that flow to the ocean in geologically distinct patterns, are closed hydrologic systems. The surface of their waters drain to inland terminal locations in which the water seeps into the ground or evaporates, having no access to excrete into the sea. Endorheic bodies of water include some of the largest lakes in the world, for example, the Aral Sea and the Caspian Sea, which is the world’s largest saline water body that is cut off from the ocean.
The endorheic regions can come about in any climate, but are most commonly found in desert settings. In regions where the rainfall is higher, riparian erosion will usually carve out the drainage channels, especially in times of flood, or initiate the rising of the water level in the terminal lake until it finds an opening, breaking the enclosed endorheic hydrological system’s geographical barricade and opening it to the adjoining terrain. The Black Sea was possibly such a lake, having once been an independent hydrological system before the Mediterranean Sea breached the terrain, dividing the two.
These endorheic areas have a tendency to be far inland with their perimeters marked by mountains or other geological features that serve as a blockade to the access to oceans. Given that the inflowing water can vacate through evaporation or seepage only, dried minerals or other materials are collected in the basin, inevitably creating the water saline and making the basin vulnerable to pollution as well. Continents fluctuate in their concentration of endorheic areas because of the conditions of geography and the climate. Australia has the highest percentage of endorheic areas at 21 percent while North America holds the least at 5 percent. Roughly 18 percent of the earth’s land empties into endorheic seas or lakes, the largest of these land regions being the interior of Asia.
In the deserts, the water inflow is low and the loss to solar evaporation is high, significantly reducing the creation of whole drainage systems. Closed water flow areas frequently lead to the concentration of salts and other minerals in the basin. Minerals leached from the neighboring rocks are placed in the basin, and left behind when the water evaporates. Therefore, endorheic basins frequently include widespread salt pans. These areas are likely to be large and flat hardened surfaces and are occasionally utilized for aviation runways or land speed record attempts, due to their broad regions of perfectly level terrain.
Both seasonal and permanent endorheic lakes can be formed in endorheic basins. Some endorheic basins are basically stable, climate change having reduced the precipitation to the extent that a lake no longer forms. Even the majority of permanent endorheic lakes significantly change in size and shape over time, often becoming much smaller or breaking into several smaller portions during the dry season. Due to humans expanding into formerly uninhabitable desert regions, the river systems that feed numerous endorheic lakes have been distorted by the building of dams and aqueducts. Consequently, many endorheic lakes located in developed or developing countries have tapered off drastically, resulting in an increase in salinity, the disturbance of ecosystems, and higher concentrations of pollutants.
Even with the exorheic basins, there can be “non-contributing”, low lying vicinities that trap the runoff and thwart the contribution of flowing downstream during the years of average or below-average runoff. In level river basins, non-contributing regions can be a large portion of the river basin.
Image Caption: Tarim Basin, Asia. Credit: NASA/Wikipedia