Headward erosion is erosion that lengthens a stream, a valley or a gully at its head and also enlarges its drainage basin. If headward erosion continues long enough, it can cause a stream to break through into a neighboring watershed and capture drainage that previously flowed to another stream. For example, headward erosion by the Shenandoah River in the U.S. state of Virginia, a tributary of the Potomac River, permitted the Shenandoah to capture successively the original upstream segments of Beaverdam Creek, Gap Run and Goose Creek, three smaller tributaries of the Potomac.