1 ADJUSTMENT OF THE BED SURFACE SIZE DISTRIBUTION OF GRAVEL-BED RIVERS IN RESPONSE TO CYCLED HYDROGRAPHS

Mountain gravel-bed rivers typically display a surface layer that is armored. That is, the surface layer visible at low flow is coarser than both the substrate and mean annual bedload transported. The surface layer is difficult to sample at the high flows that transport most of the gravel. As a result, the question as to whether the surface layer remains armored at high flows is something of a mystery. The few measurements available suggest that some form of armoring may be in place at high flows as well. In lieu of more measurements, numerical modelling provides an avenue to explore this issue. Research results are presented using a 1D model of aggradation and degradation to mobile-bed equilibrium in gravel-bed streams. In the model, a hydrograph is cycled repeatedly so that water discharge goes up and down in time. The magnitude of the bedload feed rate and the size distribution of the feed material are, however, held constant at the upstream end of the reach. As a result, the final mobile-bed equilibrium attained is characterized by a bed at the upstream end of the reach that cyclically degrades and coarsens at high flow (when the sediment feed rate is not sufficient) and aggrades an