Conventional equations for predicting alluvial channel depths and velocities are primarily based on theory and experiments with channel boundaries which are rigid (rough) and impermeable. These equations rely on calibration of a reach-specific coefficient or a relationship between hydraulic roughness and a physical dimension of the elements forming the channel boundary. For sand and gravel beds the boundary is often permeable and, on the basis of near-hydrostatic pressure occurring within pores of the channel bed, a simple ＇coefficient of friction＇ how resistance model is developed. Data from rivers and flumes With loose bed materials confirm this model and show that bed shear stress can be related to hydraulic roughness by a friction coefficient. Higher correlation is found between hydraulic roughness and bed shear stress than found for the more conventional relation between hydraulic roughness and a representative bed-material size. The suggested model is applied to 67 gravel bed reaches in Canada. The model gives better predictions of measured mean velocity than are obtainable with conventional flow resistance equations which are based on a bed-material size.