The process by which particles of river bed material are sorted into sections of near-uniform particle size due to the change in river competence throughout its journey from source to mouth; see Attal and Lave (2006) Spec. Pap. Geol. Soc. Am. 398, 143. Hydraulic sorting also occurs on beaches; try Hughes et al. (2000) J. Sed. Res. 70, 5, 994.