ABSTRACT
The morphology of natural river channels depends on the interaction between the fluid flow and the erodible materials in the channel boundary. The underlying problem is to understand that interaction, given that it involves the distinct processes of entrainment, transport and deposition of sediment. The basic mechanical principles are well established but a complete analytical solution is still a long way off, largely because natural streams represent the movement of a fluid-solid mixture within boundaries that are themselves deformable. Fluid-boundary interactions occur over a wide variety of spatio-temporal scales, from the instantaneous motion of single particles to the long-term transport of bed material along a river. Given the highly variable character of natural river flow, the channel boundary has to withstand and adjust to a large range of forces if it is to be maintained as a coherent structure.
