Material moved along a river bed by traction and saltation. Bed load usually comprises sands and pebbles, but when the water level is high and the current strong, large material boulders may be moved (Laronne (2001) Water Resources Res. 37, 5). Recking et al. (2012, ESPL 37, 774) describe bed-load transport as ‘a highly fluctuating temporal phenomenon, even under constant (mean) flow conditions, as a consequence of stochasticity, bedform migration, grain sorting, hysteresis, or sediment supply limitation’, and consider how timescale can affect bed-load prediction.