Bed particle dispersion and scour in the San Juan (Pacheedaht) River

As part of a larger river restoration project, bed particle dispersion was tracked at pool tailouts where salmon spawning is known to occur in the San Juan (Pacheedaht) River, B.C. Mobility and pathlengths of surface and sub-surface radio-tagged particles was characterized in three reaches over two deployment years in relation to annual channel change, flow magnitude, bar morphology and particle size. Surface particle mobility was high, with results from subsurface tracers indicating high spatial variation of scour at both the reach and local scale. Results also suggest scour can occur up to depths of 0.3m at some locations, even during moderate flood events. Trapping areas of tracer clusters saw consistent annual deposition and can be tied to overall bar development and annual reach-scale channel change. Even during extreme flood events, tracer pathlengths rarely exceeded one riffle-pool-bar unit, further validating the role of channel morphology, along with flow, on particle dispersion