thalweg definition

• watershed narratives
thalweg1
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 one crossing all contour lines at right angles 

c  :  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 

Origin of THALWEG

German talweg (formerly spelled thalweg), from tal valley (from Old High German) + weg way, path, from Old High German — more at dale, way
Merriam-Webster