Thalweg

[ tahl-veg, -veyk ]

noun

1. a line, as drawn on a map, connecting the lowest points of a valley.
2. Chiefly International Law. the middle of the main navigable channel of a waterway that serves as a boundary line between states.

https://www.dictionary.com/browse/thalweg

Guyana claimed a thalweg boundary of the Courantyne River (probably inspired by the 1962 Dutch position), and a 35° line from the true North, from the mouth of the river. Suriname claimed sovereignty over the full width of the Courantyne and a 10° dividing line.

Wikipedia: Disputed border between Guyana and Suriname. Retrieved 2024-02-09