thalweg
最深谷底線; 剖麵線
Thal·weg n. 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.
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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