Source : Webster's Revised Unabridged Dictionary (1913)

Thalweg \Thal"weg`\, n. [G., fr. thal valley + weg way. See
   {Dale}; {Way}.] (Physiography)
   (a) A line following the lowest part of a valley, whether
       under water or not.
   (b) The line of continuous maximum descent from any point on
       a land surface, or that cutting all contours and angles.

Source : WordNet®

thalweg
     n 1: the middle of the chief navigable channel of a waterway that
          forms the boundary line between states
     2: a line following the lowest points of a valley