1.  thalweg(n = noun.shape) - the middle of the chief navigable channel of a waterway that forms the boundary line between states;
is a kind of bound, boundary, edge

2.  thalweg(n = noun.communication) - a line following the lowest points of a valley;
is a kind of contour, contour line

   A line following the lowest part of a valley, whether under water or not. [Webster 1913 Suppl.]

thalweg, n.
1 Geog. a line where opposite slopes meet at the bottom of a valley, river, or lake.
2 Law a boundary between States along the centre of a river etc.

Etymology:
G f. Thal valley + Weg way