thalweg

noun

1. a line, as drawn on a map, connecting the lowest points of a valley.
2. Chiefly International Law. the middle of the main navigable channel of a waterway that serves as a boundary line between states.

noun geography rare

1. the longitudinal outline of a riverbed from source to mouth
2. the line of steepest descent from any point on the land surface

n.1831, from German Thalweg “path along the bottom of a valley,” from thal (see dale) + weg (see way).

1. The line defining the lowest points along the length of a river bed or valley, whether underwater or not.
2. A subterranean stream following a course similar to that of an overlying surficial stream.