Salmon as Geomorphic Agents in Gravel-Bed Rivers

We examined the impact of salmonid spawning activities and floods on channel morphology, streambed roughness, bed material transport, bed surface texture and stability and fine sediment dynamics of small gravel bed streams in British Columbia, Canada. We hypothesize that salmonid redds affect streambed roughness and shear stress partitioning between bed forms and grains.  Channel morphology and dynamics of a large number of streams in British Columbia are partially or wholly affected by fish bioturbation. The scale of the impact is controlled by the salmon species, population density, and channel size and characteristics. Sediment transport measurements show that salmon play a significant role in erosion and deposition within the channel by promoting vertical and longitudinal mixing of the substrate, and altering the relative mobility of the bed material. Salmon bioturbation promotes distinctive bedforms and packing of sediment grains. In streams with dense populations of sockeye or chum salmon the whole surface of spawning reaches may be modified, as bars are excavated and pools are filled. For chinook salmon the organization of spawning bedforms ranges from scattered mounds or ‘gravel pile-ups’ to well-ordered dunes. Such dunes extend for hundreds of meters to kilometres along the river bed. They exhibit amplitudes of more than one metre and wavelengths of 10 to 15 m.  We test our hypothesis with a two-dimensional hydrodynamic models (FastMECH) supported by topography with and without redds to single out the influence of this biotic formation on stream hydraulics. Simulations show that these macro forms increase streambed irregularities, which results in increase flow resistance, deeper flows and higher flow field heterogeneity. Our conclusion that mass-spawning fish can dominate sediment transport in mountain drainage basins has fundamental implications for understanding channel morphology, aquatic ecosystem dynamics, stream responses to environmental change, and river restoration programs.