When one river captures another

A ridge separating two river systems may appear permanent, yet erosion can shift this drainage divide. Each system drains water from its own area, called a basin. If one stream cuts backward into a ridge faster than a stream on the opposite side, the boundary between their basins gradually moves.

This backward extension of a channel is called headward erosion. It can occur when water wears away rock at a channel’s upper end. Given enough time, an advancing channel may reach a neighboring stream whose bed lies at a higher elevation. Water from that stream then enters the lower channel, following the steeper route. Geologists call this redirection river capture.

The receiving river gains water from an area that formerly supplied its neighbor. Greater flow can increase its ability to erode, reinforcing the process that enlarged its basin. Downstream from the capture point, the other river loses part of its supply. Its remaining flow may occupy a valley much wider than it now needs.

Such a valley is a clue to the earlier drainage pattern. An abrupt bend where the captured stream joins its new channel can provide another. Together, these landscape features help explain how a river network acquired its present shape.