What is (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 called?

thalweg — (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. (Webster's 1913.)

Sources