It is often found that a coarse surface layer exists in the outer-bank region, and a finer one in the inner-bank region in natural stream bends. This phenomenongenerally results from the sortingof graded bed load. A method for the calculation of bed load discharge is proposed, in which the effects of bed slope and flowdirection on sediment transport have been taken into accounts. Furthermore, a two-dimensional numerical simulation modelis developed, which is useful for predicting of the spatial and temporal variations of grain size distribution as well as of the bed configuration. Discussions are made on the sorting process and the effects of grain size distribution on the bed topography.