The word is of 19th-century German origin. The German word Thalweg is a compound noun that is built from the German elements Thal meaning valley, and Weg, meaning way. It literally means "valley way" and is used, with its modern spelling Talweg, in daily German to describe a path or road that follows the bottom of a valley, or in geography with the more technical meaning also adopted by English.
Hydrology
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 of a watercourse. 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.
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 artificial sills also helps to slow the sedimentary erosion and deposit of running water sources, while keeping the esteem 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 help 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 in Umatilla, Oregon.
Thalweg principle
The thalweg principle is the legal principle that if the boundary between two political entities is stated to be a waterway, without further description, the boundary follows the thalweg of that watercourse. In particular, the boundary follows the center of the principal navigable channel of the waterway. If there are multiple navigable channels in a river, the one principally used for downstream travel is used. This definition has been used in specific descriptions as well. The Treaty of Versailles, for example, specifies that "In the case of boundaries which are defined by a waterway" the boundary is to follow "the median line of the principal channel of navigation." The precise drawing of river boundaries has been important on countless occasions; notable examples include the Shatt al-Arab between Iraq and Iran, the Danube in central Europe, the Kasikili/Sedudu Island dispute between Namibia and Botswana, 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.
