Populations of Pacific salmon species have declined sharply in many rivers of the western US. Reversing these declines is a top priority and multimillion- dollar-per-year expense of many river restoration projects. To help restore salmon populations, managers often inject spawning-sized gravel into rivers. However, there is little quantitative understanding of how bed coarsening affects salmon spawning, and there little mechanistic basis concerning what the salmon actually need for spawning. Furthermore, there is little quantitative basis for determining where added gravel would be most suitable. The proposed project seeks to provide such a basis by systematically quantifying the upper, particle-size limits of spawning gravel for pink and chinook salmon (Oncorhynchus gorbuscha and O.tshawytsha). The work is founded on a series of mechanistic hypotheses about the spawning suitability of natural gravels and it will explore these hypotheses quantitatively with new measurements of fish size, redd dimensions, flow velocity, and grainsize characteristics of the bed for spawning pink and chinook salmon, the smallest and largest of the Pacific salmon species. This project focuses on rivers where salmon returns are high and substrate is coarse: the Puyallup River system (Puget Sound), where pink salmon are expected to return in exceptionally high numbers in fall 2009, and the Trinity River (coastal California), where 2009 fall-run chinook salmon are expected to return in high numbers. Because the lead time to the peak of spawning is short, a rapid response field effort will capitalize on this coincidence of large runs of salmon in coarse reaches of river. Strong relationships are expected between fish size and largest particle moved and between redd building success and percent coverage of immovable particles.
This project will contribute to increased efficiency of gravel augmentation projects and improve the long-term efficacy of many river restoration efforts. By promoting effective river restoration, this project may help improve the survival outlook for some of the threatened and endangered species of Pacific salmon. The project cultivates an ongoing collaboration between the academic investigator and members of the ecosystem restoration consulting industry.
