A more thorough explanation:

Definition: Thalweg (pronounced tahl-vayk or -veg) refers to a line that follows the lowest part of a valley, which is usually submerged. It can also refer to the middle of the primary navigable channel of a waterway, which serves as the boundary between states.

Example: In the case of Louisiana v. Mississippi, the thalweg was used to determine the boundary between the two states. The thalweg is the part of the river channel that is most followed, usually at the middle of the principal channel.

Explanation: The example illustrates how the thalweg is used to determine the boundary between states. The thalweg is the line that follows the lowest part of the river valley, which is usually the middle of the primary channel. In the case of Louisiana v. Mississippi, the thalweg was used to determine the boundary between the two states because it is the most followed part of the river channel.