Headward erosion

Once a stream has begun to cut back, the erosion is sped up by the steep gradient the water is flowing down.

For example, headward erosion by the Shenandoah River, a tributary of the Potomac River in the U.S. state of Virginia, permitted the Shenandoah to capture successively the original upstream segments of Beaverdam Creek, Gap Run and Goose Creek, three smaller tributaries of the Potomac.

Insequent streams form by random headward erosion, usually from sheetflow of water over the landform surface.

The water collects in channels where the velocity and erosional power increase, cutting into and extending the heads of gullies.

Subsequent streams form by selective headward erosion by cutting away at less resistive rocks in the terrain.

Headward erosion is demonstrated in this 1906 photo taken near Mount Tamalpais in Marin County, California . Groundwater sapping is causing this gully to lengthen up the slope.
At Canyonlands National Park in Utah , sheets of water flow across the plains and enter the canyons. As they do so, the top edge of the canyon erodes back into the plain above, both lengthening and widening the canyon. Widening of a canyon by erosion inside the canyon below the top, because of the flow of the stream inside the canyon, is not called headward erosion.
Active headward erosion ( lavaka ) on right, with inactive, largely infilled older examples to left