This paper first describes the phenomenon of channel widening during degradation of alluvial rivers,explains the mechanisms of channel widening, and analyzes the stability of cohesive riverbank. Then a one-dimensional mathematical model is developed to simulate the transport of non-uniform suspended sediments, with a sub-model for the simulation of channel widening, and is used to study the process of channel widening during degradation. The effects of different incident flow and sediment conditions and different riverbank material characteristics on channel widening and bed degradation are compared.Finally, main factors that control the deformation processes are identified.