Thalweg

Thalweg (pronEng|ˈtɑːlvɛg [OED] ; an English word compounded from the German elements "Thal" (since Duden's orthography reform of 1901 written "Tal") meaning "valley", and "Weg", meaning "way") is a term adopted into English usage for geography and geomorphology. It signifies the deepest continuous line along a valley or watercourse.

In hydrology, the thalweg (sometimes called the "valley line") is a line drawn to join the lowest points along the entire length of a streambed or valley in its downward slope, defining its deepest channel. It thus marks the natural direction (the profile) of a watercourse. The thalweg is almost always the line of fastest flow in any river. The term is also sometimes used to refer to a subterranean stream that percolates under the surface and in the same general direction as the surface stream.

In geomorphology, the thalweg is the continuous line that follows the path of maximum slope or descent between two points. As such, it is at right angles to all contour lines. This definition encompasses the line connecting the lowest points along the valley floor or bed of a stream.

The thalweg principle is the principle which defines the border between two states separated by a watercourse as lying along the thalweg. The precise drawing of river borders has been important on countless occasions; notable examples include the Shatt al-Arab (known as Arvand Rud in Iran) between Iraq and Iran, the Danube in central Europe, the Kasikili/Sedudu Island dispute between Namibia and Botswana, settled by the International Court of Justice in 1999. [http://www.icj-cij.org/docket/index.php?case=98&p1=3&p2=1&p3=6&pr=318&search=%22botswana%22] and the 2004 dispute settlement under the UN Law of the Sea concerning the offshore boundary between Guyana and Surinam, South America, in which the thalweg of the Corentyne River played a role in the ruling.

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