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In hydrological and fluvial landforms, the thalweg is a line drawn to join the lowest points along the entire length of a stream bed or valley in its downward slope, defining its deepest channel. The thalweg thus marks the natural direction (the profile) of a watercourse. The thalweg is almost always the line of fastest flow in any river.
The word derives from German "Talweg" (old spelling "Thalweg"), meaning "valley way".
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.
Real world application 
Slowing stream bed erosion by taking advantage of a thalweg helps stabilize running water sources. Placing boulders along the thalweg in a running water source helps to protect the channel's sedimentary erosion and deposit balance. In concurrence with the placement of boulders along a thalweg, the placement of boulders along an instream to form man-made sills also helps to slow the sedimentary erosion and deposit of running water sources, while keeping the esteem (fishing, local wildlife, and recreation) and natural resources  of the running water source intact. The placement of boulders along a thalweg and the creation of instream sills help to aid the flow of water during late summer months when the flow rate drops, and helps to slow sedimentary erosion and deposit in the spring and fall months when the flow rates are high. This process of utilizing a thalweg to slow stream bed erosion was used in Meacham Creek, Umatilla, Oregon (1990).
Thalweg principle 
The thalweg principle is the principle in which the boundary between two political states separated by a watercourse is denoted as the thalweg of that watercourse, if those two states have agreed to use the thalweg definition. Various states have also defined their watercourse international boundaries by a median line, left bank, right bank, etc.
The precise drawing of river boundaries 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, and the 2004 dispute settlement under the UN Law of the Sea concerning the offshore boundary between Guyana and Suriname, in which the thalweg of the Courantyne River played a role in the ruling.
Thalweg is an English loan word from German. The German word Thalweg (nowadays spelt Talweg) is a compound noun that is built from the German elements Thal (since Duden's orthography reform of 1901 written Tal) meaning valley, and Weg, meaning way.
See also 
- Oxford English Dictionary
- Webster Dictionary
- Umatilla River Basin Anadromous Fish Habitat Enhancement Project 1990 Annual Report Carl A. Sheeler, Fish Habitat Biologist Slatick, January 1991
- "Kasikili/Sedudu Island (Botswana/Namibia)". International Court of Justice. 13 December 1999. Retrieved 10 February 2012.