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NASA photo of the endorheic Tarim Basin
Endorheic basin showing waterflow input

An endorheic basin [1] (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.[2] 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.[3]

Contents

Endorheic lakes

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. 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.[4]

Endorheic lakes are usually in the interior of a body mass, far from an ocean. Their watersheds are often confined by natural geologic land formations such as a mountain range, cutting off water access to the ocean. The inland water flows into dry watersheds where the water evaporates, leaving a high concentration of minerals and other inflow erosion products. Over time this input of erosion products can cause the endorheic lake to become relatively saline (a "salt lake"). Since the main outflow pathways of these lakes are chiefly through evaporation and seepage, endorheic lakes are usually more sensitive to environmental pollutants inputs than water bodies that have access to oceans.[3]

Occurrence

Endorheic regions can occur in any climate but are most commonly found in hot desert locations. In areas where rainfall is higher, riparian erosion will generally carve drainage channels (particularly in times of flood), or cause the water level in the terminal lake to rise until it finds an outlet, breaking the enclosed endorheic hydrological system’s geographical barrier and opening it to the surrounding terrain. The Black Sea was likely such a lake, having once been an independent hydrological system before the Mediterranean Sea broke through the terrain separating the two.

Endorheic regions tend to be far inland with their boundaries defined by mountains or other geological features that block their access to oceans. Since the inflowing water can evacuate only through seepage or evaporation, dried minerals or other products collect in the basin, eventually making the water saline and also making the basin vulnerable to pollution.[3] Continents vary in their concentration of endorheic regions due to conditions of geography and climate. Australia has the highest percentage of endorheic regions at 21 percent while North America has the least at 5 percent.[5] Approximately 18 percent of the earth’s land drains to endorheic lakes or seas, the largest of these land areas being the interior of Asia.

In deserts, water inflow is low and loss to solar evaporation high, drastically reducing the formation of complete drainage systems. Closed water flow areas often lead to the concentration of salts and other minerals in the basin. Minerals leached from the surrounding rocks are deposited in the basin, and left behind when the water evaporates. Thus endorheic basins often contain extensive salt pans (also called salt flats, salt lakes, alkali flats, dry lake beds or playas). These areas tend to be large, flat hardened surfaces and are sometimes used for aviation runways or land speed record attempts, because of their extensive areas of perfectly level terrain.

Both permanent and seasonal endorheic lakes can form in endorheic basins. Some endorheic basins are essentially stable, climate change having reduced precipitation to the degree that a lake no longer forms. Even most permanent endorheic lakes change size and shape dramatically over time, often becoming much smaller or breaking into several smaller parts during the dry season. As humans have expanded into previously uninhabitable 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, resulting in increased salinity, higher concentrations of pollutants, and the disruption of ecosystems.

Notable endorheic basins and lakes

Major endorheic basins of the world. Basins are shown in dark gray; major endorheic lakes are shown in black.
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Antarctica

Endorheic lakes in Antarctica are located in the McMurdo Dry Valleys, Victoria Land, Antarctica, the largest ice-free area in Antarctica.

  • Don Juan Pond in Wright Valley is fed by groundwater from a rock glacier and remains unfrozen throughout the year.
  • Lake Vanda in Wright Valley has a perennial ice cover, the edges of which melt in the summer allowing flow from the longest river in Antarctica, the Onyx River. The lake is over 70 m deep and is hypersaline.
  • Lake Bonney is in Taylor Valley and has a perennial ice over and two lobes separated by the Bonney Riegel. The lake is fed by glacial melt and discharge from Blood Falls. Its unique glacial history has resulted in a hypersaline brine in the bottom waters and fresh water at the surface.
  • Lake Hoare, in Taylor Valley, is the freshest of the Dry Valley lakes receiving its melt almost exclusively from the Canada Glacier. The lake has an ice cover and forms a moat during the Austral summer.
  • Lake Fryxell, in adjacent to the Ross Sea in Taylor Valley. The lake has an ice cover and receives its water from numerous glacial meltwater streams for approximately 6 weeks out of the year. Its salinity increases with depth.

Asia

Caspian Sea, a giant inland basin

Much of western and Central Asia is a single, giant inland basin. It contains several lakes, including:

Australia

Australia, being very dry and having exceedingly low runoff ratios due to its ancient soils, has a great prominence of variable, endorheic drainages. The most important are:

A false-colour satellite photo of Australia’s Lake Eyre
Image credit: NASA’s Earth Observatory

Africa

North and Central America

Great Salt Lake, Satellite photo (2003) after five years of drought

Many small lakes and ponds in North Dakota and Manitoba are endorheic; some of them have salt encrustations along their shores.

Europe

All these lakes are drained, however, either through manmade canals or via karstic phenomena. Minor additional endorheic lakes exist throughout the Mediterranean countries Spain (e.g. Laguna de Gallocanta), Italy, Cyprus (Larnaca and Akrotiri salt lakes) and Greece.

South America

MODIS image from November 4, 2001 showing Lake Titicaca, the Salar de Uyuni, and the Salar de Coipasa. These are all parts of the Altiplano

Ancient

Some of the Earth’s ancient endorheic systems include:

See also

References

  1. ^ This term is little-used among North American geoscientists.
  2. ^ "Drainage systems". Encyclopedia Britannica. http://www.britannica.com/eb/topic-187043/endorheic-system. Retrieved 2008-02-11.  
  3. ^ a b c "Endorheic Lakes: Waterbodies That Don't Flow to the Sea". United Nations Environment Programme. http://www.unep.or.jp/ietc/publications/short_series/lakereservoirs-2/10.asp. Retrieved 2008-02-11.  
  4. ^ "What is a watershed and why should I care?". university of delaware. http://www.wr.udel.edu/cb/whatwhycare.html. Retrieved 2008-02-11.  
  5. ^ "Saline Lake Ecosystems of the World". Springer. http://books.google.com/books?id=NOdvPFm6SyoC&pg=PA18&lpg=PA18&dq=endorheic&source=web&ots=EWyVvVeOhm&sig=tNrylNMQCGu6bUCBTyo1HHlGDRc. Retrieved 2007-07-31.  

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