In geography and fluvial geomorphology, a thalweg or talweg () is the line of lowest elevation within a valley or watercourse. Under international law, a thalweg is taken to be the middle of the primary navigable channel of a waterway that defines the boundary line between entities such as states. Thalwegs can acquire special significance because disputed river borders are often deemed to run along the river's thalweg. Etymology The word thalweg is of 19th-century German origin. The German word (modern spelling ) is a compound noun that is built from the German elements (since Duden's orthography reform of 1901 written ) meaning valley (cognate with dale in English), and , meaning way. It literally means "valley way" and is used, with its modern spelling , 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 (the profile) 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. 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 artificial 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 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 (also known as the thalweg doctrine or the rule of thalweg)