Ingeography,hydrography, andfluvialgeomorphology, athalweg ortalweg (/ˈtɑːlvɛɡ/) is the line or curve of lowestelevation within avalley orwatercourse.[1] Normally only thehorizontal position of the curve is considered (as viewed on amap); the correspondingvertical position is represented in astream profile.
Underinternational law, a thalweg is instead taken to be the middle of the primarynavigable channel of awaterway which is the default legal presumption for theboundary between entities such asstates.[2][3][4] Thalwegs can have local proprietorial and administrative significance because their formerly somewhat shifting position, reliant on renewed soundings, now more fixed as described internationally, is part of centuries-old custom and practice in some jurisdictions. In some jurisdictions and between some states the median line (between banks) is the preferred boundary presumption as may extend from estuaries. Also being easy to map, drawing "turning points" are the solution for a few major rivers such as theSt Lawrence River-Great Lakes system.[5]
The wordthalweg is of 19th-century German origin. The German wordThalweg (modern spellingTalweg) is a compound noun that is built from theGerman elementsThal (sinceDuden'sorthography reform of 1901 writtenTal) meaning 'valley' (cognate withdale in English), andWeg, meaning 'way'. It means 'valley way' and is used, with its modern spellingTalweg, in daily German to describe a path or road which follows the bottom of a valley, or in geography with the more technical meaning also adopted by English.
Inhydrological andfluvial landforms, the thalweg is a line drawn to join the lowest points along the length of astream bed orvalley in its downward slope, defining its deepest channel. The thalweg thus marks the natural direction (the profile) of a watercourse. The term is sometimes used to refer to a subterranean stream that percolates under the surface and in the same general direction as the surface stream.
Slowing stream-bed erosion by bouldering a thalweg helps stabilize natural rivers' course and depth. Placing boulders along the thalweg helps to protect the channel's sedimentary erosion and deposit balance. In concurrence with this, doing so along aninstream to form artificial sills helps to slow the sedimentary erosion and deposit of watercourses, while keeping the esteem (fishing, local wildlife, and recreation) and natural resources[6] of the running water source intact. Placement of boulders along a thalweg and the creation of instream sills makes drying up rarer and less severe during late summer, and abates cases of severe sediment erosion and deposit in the spring and fall months when the flow rates are high, particularly if those rates have increased. Such partial infilling of a thalweg was prototyped in Meacham Creek inUmatilla, Oregon.[7]
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Thethalweg principle (also known as thethalweg doctrine or therule of thalweg) is the legal principle that if theboundary between two political entities is stated to be a waterway without further description (e.g., a median line, right bank, eastern shore, low-tide line, etc.), the boundary follows the thalweg of that watercourse. A thalweg is the center of the principal navigable channel of the waterway (which is presumed to be the deepest part).[8] If there are multiple navigable channels in a river, the one principally used for downstream travel (likely having the strongest current) is used.[8] The definition has been used in specific descriptions as well. TheTreaty of Versailles, for example, specifies that "In the case of boundaries which are defined by a [navigable] waterway" the boundary is to follow "the median line of the principal channel of navigation."[9]
The precise drawing of river boundaries has been important on countless occasions. Notable examples include theShatt al-Arab betweenIraq and Iran, theDanube in centralEurope (Croatia–Serbia border dispute), theKasikili/Sedudu Island dispute betweenNamibia andBotswana (settled by theInternational Court of Justice in 1999),[10] and the 2004 dispute settlement under the UNLaw of the Sea concerning the offshore boundary betweenGuyana andSuriname, in which the thalweg of theCourantyne River played a role in the ruling.[11][12] In the 20th century dispute between the USSR and China (PRC) overZhenbao Island,China held that the Thalweg principle supported their position.[13] The doctrine is also applied to sub-national boundaries, such as those between American states.[14]
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