Based on the momentum equation of flow, the force per unit area which the flow exerts radially on the outer bank of a stream bend is expressed by an equation in which the force is explicitly related to the curvature ratio R/w (curve radius/channel width). A dimensionless curvature coefficient is defined, which is dependent on R/w and the coefficient of dynamic bedload friction, . Exploring the changes of with R/w reveals a functional relationship which, for the known values of , shows maximal values of for R/w values between 2 and 4. The theoretical results agree with field measurements of the relationship between R/w and the rate of channel migration.