1. The middle of the chief navigable channel of a waterway that forms the boundary line between states
   "The thalweg of the Mississippi River serves as the border between several states"
    
2. A line following the lowest points of a valley
   "The thalweg marked the deepest part of the river channel"

Derived forms: thalwegs

Type of: bound, boundary, contour, contour line, edge

Encyclopedia: Thalweg