A site on the long profile of a river where water falls vertically. Waterfalls may be found at a band of more resistant rock (Alexandrowicz (1994) Geomorph. 9, 2), at a knick point (Crosby and Whipple (2006) Geomorph. 82, 1–2), or where deposition has occurred. Hyakawa and Matsukora (2003) ESPL 28, 6 note that the principal factor in determining the rate of waterfall recession is the ratio of the erosive force of stream to the bedrock resistance, and derive an empirical equation connecting the force/resistance ratios and the rates of waterfall recession.