thalweg
noun
thal·weg
| \ ˈtälˌveg, -vāḵ\
plural -s
Definition of thalweg
1a
: 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
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