Longitudinally undulating gravel or boulder accumulations that act as local steps. Irregular spacing is dictated by the distribution of bedrock outcrops, wood or hillslope sediment inputs along the river. They tend to occur at wider sections of valley in bedrock-confined systems (e.g. at tributary confluences).
Flow is characterised by high energy turbulence over lobate accumulations of coarse bedload materials, wood and bedrock outcrops. At the lower end of the energy spectrum, riffle-pool spacing in bedrock-confined settings may reflect purely rhythmic hydraulic processes of sediment transport.