Thalweg

In geography, hydrography, and fluvial geomorphology, a thalweg or talweg (/ˈtɑːlvɛɡ/) is the line or curve of lowest elevation within a valley or watercourse.^[1] Its vertical position in maps is the nadir (greatest depth, sounding) in the stream profile.

Under international law, a thalweg is instead taken to be the middle of the primary navigable channel of a waterway which is the default legal presumption for the boundary between entities such as states.^[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 the St Lawrence River-Great Lakes system.^[5]