Endorheic

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In geography, an endorheic basin is a watershed from which there is no outflow of water (either on the surface as rivers, or underground by flow or diffusion through rock or permeable material)(also called closed basin). The term has Greek roots, endo, "inside" and rhein, "to flow". Any rain (or other precipitation) that falls inside an endorheic basin remains there permanently, leaving the system only by evaporation. Endorheic basins are also called "internal drainage systems".

Although, in theory, endorheic basins can occur in any climate, in practice they are most commonly found in hot desert locations. In places with a higher rainfall, the riparian erosion of the water's flow will generally carve drainage channels (particularly in times of flood), breaking through to the larger enclosing hydrological system, and breaking the watershed barrier between the endorheic system and the surrounding terrain. The Black Sea was such a lake, having once been an independent hydrological system in its own right before the Mediterranean Sea broke through the terrain separating the two.

In hot deserts, the net inflow is low and loss to solar evaporation high, drastically slowing the formation of complete drainage systems. The closed nature of this water flow often leads to the concentration of salts and other minerals in the lake; minerals leached from the surrounding rocks are deposited in the endorheic basin, and left behind when the water that bore them there evaporates. Thus endorheic basins often contain extensive salt pans (also called salt flats, alkali flats or playas), salt lakes. These areas, which tend to be large, hard surfaced, and fairly flat, are sometimes used for aviation (as large cheap runways) or for breaking land speed records.

Both permanent and seasonal endorheic lakes can form in endorheic basins, and some endorheic basins are essentially in stasis, the climate having changed to reduce precipitation to such an extent that a lake no longer forms. Even with endorheic lakes that exist permanently, most change size and shape dramatically over time, often becoming dramatically smaller (or breaking into several smaller parts) during the dry season. As humans have expanded into previously unliveable desert areas, the river systems that feed many endorheic lakes have been altered by the construction of dams and aqueducts. As a result many endorheic lakes in developed or developing countries have contracted dramatically. This often results in dramatic increases in salinity, higher concentrations of pollutants, and the consequent disruption of the lake's ecosystem.

Image:NEO lake eyre big.jpg

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Notable endorheic basins and lakes

Much of western and Central Asia is a single, giant inland basin. It contains a number of lakes, including:
- The Caspian Sea is the largest lake on Earth.
- The Aral Sea's tributary rivers have been diverted, leading to a dramatic shrinkage of the lake. The resulting ecological disaster has brought the plight faced by internal drainage basins to public attention.
- Lake Balkhash is located in Kazakhstan.
- Issyk Kul is located in Kyrgyzstan.
The Dead Sea is located in the Middle East.
Australia's Lake Eyre Basin, the largest endorheic system in the world, includes much of the landmass of the arid inland, and drains into the highly variable Lake Eyre.
Africa also has many endorheic watersheds.
- Lake George
- The Okavango Delta, an endorheic inland delta in the Kalahari Desert of Botswana
- Lake Ngami in Botswana
- Lake Chad (between Chad and Cameroon), fed by the Chari and Logon rivers
- Etosha pan in Namibia's Etosha National Park
One of the few endorheic lakes in a cold desert location, Antarctica's Lake Vida remains liquid because its salinity is seven times that of seawater.
The US's Great Basin, which covers much of Nevada and Utah, and includes:
- The Black Rock Desert in Nevada, location of the Thrust2 and ThrustSSC landspeed record runs, and the annual home to the Burning Man festival
- Groom Dry Lake in Nevada, location of the secret Area 51 base
- Utah's Great Salt Lake
- Pyramid Lake in Nevada
- Mono Lake in California
California's Salton Sea, a lake accidentally created in 1905 when irrigation canals ruptured, filling a desert endorheic basin and recreating an ancient saline sea.
The Great Divide Basin in Wyoming, a small endorheic basin which straddles the Continental Divide.
Crater Lake in Oregon.
The Valley of Mexico. In Pre-Columbian times, the Valley was substantially covered with five lakes, including Lake Texcoco and Lake Chalco.

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Ancient endorheic basins

Some of the Earth's ancient endorheic systems include:

the Black Sea, until its merger with the Mediterranean
Lake Lahontan in the western US
Ebro and Duero basins, draining most of northern Spain during the Neogene and perhaps Pliocene.
Lake Bonneville (Utah)