The lengthening of a river's course, by erosion back towards its source. A knick point on a stream profile suggests that the fluvial system has reacted to a lower base level by headward erosion (Schlunegger and Schneider (2005) Geomorph. 69, 1–4). See also Hill et al. (2008) Geomorph. 95, 3–4 on the Grand Canyon, USA.