How fast and how far do variable boundary conditions affect river morphodynamics?

The analytical solution of an one-dimensional morphodynamic model of a river reach with nonuniform grain size material under unsteady flow conditions is discussed. The model Harmonic River assumes an instantaneous propagation of the water flow along the river reach, a sinusoidal variation of the boundary conditions at both ends in time, as well as of the channel width in space, by accounting for two grain size classes of solid material. Apart from the instantaneous propagation of water flow, all information regarding the boundary conditions are conveyed along the river reach by three waves. Except for extremely low Froude numbers which may occur typically for dammed rivers, and for extremely long wave period (geological time scales), both the second and third waves may be neglected as virtually all information is transmitted by the first wave into the downstream direction. Moreover, the dominant boundary conditions affecting the river reach are represented by both the sediment transport rate and the sediment transport composition.