noun 1 : subsurface water percolating beneath and in the same direction as a surface stream course 2 : the middle of the chief navigable channel of a waterway which constitutes a boundary line between states
While thalweg more famously (though still not actually famously) refers to a line that follows the lowest part of a valley (whether or not water is present), and even somewhat less famously to a line of continuous maximum descent from any point on a land surface, or to a line crossing all contour lines at right angles, its water-specific meanings are what earns the word a place here. Thank the Germans for the word's existence: it's a German borrowing, from Tal, meaning "valley," and Weg, meaning "way, path."