Master of Science
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.
Summary for Lay Audience
Human influences have greatly impaired the ecological health of rivers and has led to a global focus on improving the scientific basis for river restoration, conservation, and management practices. Well-informed decisions on restoration and management plans for a river should include knowledge of the rate at which sediment moves through the river system, also referred to as bedload transport. Bedload transport is of specific importance because it is fundamental to shaping river channels and structuring depositional features (e.g. banks and bars). This can affect flooding risks, in-channel flow velocity and depth, physical habitat, and the river’s response to changes in land use and other conditions in the watershed. The goal of this study was to investigate bedload transport processes near salmon spawning habitat through individual particle tracking to better understand the influences of channel morphology, flow, and grain size on particle dispersion. To achieve this, radio-activated tracer stones were deployed along the surface and subsurface (buried) at three study reaches over two deployment years, on the San Juan (Pacheedaht) River, B.C., a large wandering gravel-bed river. In addition, annual change in channel morphology was mapped using aerial imagery. Results show high mobility of surface tracers at known salmon spawning habitat with scour (erosion) at some locations occurring up to, and probably exceeding, 0.3m depths even during years of moderate flood events. Individual pathlength distances of tracer particles rarely exceeded one riffle-pool-bar unit. Areas of high tracer deposition can be linked to bar development and observations of annual channel change. Overall, results indicate that deep scour may be a risk to salmon spawning habitat, as well as implications on the overall stability of banks and bars in relation to flow and sediment supply, providing key information to support ongoing restoration work on the San Juan River.
Sauro, Meghan, "Bed particle dispersion and scour in the San Juan (Pacheedaht) River" (2021). Electronic Thesis and Dissertation Repository. 8316.