The Collaborative International Dictionary of English v.0.48:

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.
       [Webster 1913 Suppl.] Thammuz

WordNet (r) 3.0 (2006):

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