Bagnold's bed load transport model is reexamined by using only the momentum equation, the Coulomb yield criteria and the definition of the rate of energy dissipation for a shearing fluid. The general form of Bagnold's bed load equation is rederived but with two specific differences: Coulomb's yield criteria tan ϕ=T/P holds only at the interface between regions of shearing and nonshearing grains, and the ratio of the mean to maximum granular velocity U/Umax enters as an additional factor.