1. A line following the lowest part of a (usu. submerged) valley.
2. The middle of the primary navigable channel of a waterway, constituting the boundary between states. — Also spelled talweg.
— Also termed midway. [Cases: States 12. 1.]
live thalweg. The part of a river channel that is most followed, usu. at the middle of the principal channel. Louisiana v. Mississippi, 466 U.S. 96, 104 S.Ct. 1645 (1984).