The thalweg (also spelled talweg), meaning “valley way” in German, is the line on a topographic map that connects the lowest points in a river channel—an abstract vector that predicts various features of the watercourse. Water velocity will be greater in the thalweg, because there is less friction where the bed is deepest. As Rebecca Lawton writes in Reading Water: Lessons from the River, “In flatwater the thalweg will carry a boat downstream faster than rowing. In rapids, it will find a way between boulders or through the deepest, safest channel nine times out of ten.” Where the river cuts into its bank, the thalweg will be closer to the bank than to the middle of the channel. Less commonly, the thalweg is the subterranean portion of a stream. In law, the thalweg is the invisible divide through the center of the deepest channel of a border river, thus assuring the navigation rights of both suspicious parties. When a boundary river shifts, the boundary can remain in the thalweg of the original channel, even though it’s now dry, such as along portions of the Rio Grande between the United States and Mexico, and the Colorado River between California and Arizona.

D. J. Waldie