KEY RESEARCH NEEDS IN GRAVEL AUGMENTATION

Gravel augmentation has become a widely-used river restoration strategy, particularly in California's Central Valley where upstream dams have blocked the supply of coarse sediment to downstream reaches and have greatly reduced the frequency bed-mobilizing high flows. Coarse sediment additions are commonly intended to improve habitat for spawning salmonids by altering stream bed grain size distributions, increasing the frequency of bed mobilization, and the diversity of channel morphology. The effectiveness of gravel augmentation is currently limited by a lack of science-based methodologies for predicting both the spatial and temporal extent of geomorphic change that might be caused by specific quantities and frequencies of coarse sediment additions, and the potential consequent beneficial effects for salmonids in particular and riverine ecosystems in general. Here we identify a set of key geomorphic and biological research questions, many of which could be addressed through collaborations between academic researchers and restoration practitioners in an experimental framework within ongoing and future river restoration projects.

Key geomorphic questions include: How can we predict the spatial and temporal effects of gravel augmentation on channel form? Can addition of fine gravel be used to mobilize an immobile, armored coarse surface layer? Can gravel additions be designed to result in removal of interstitial sands and silts? Key biological questions include: What factors influence salmonid use of introduced gravel for spawning? Does gravel augmentation influence the risk of redd scour? Can gravel augmentation improve non-spawning habitat for salmonids, or create secondary benefits for other species?

Many key issues integrate geomorphic and biological questions, including: What are the quantitative linkages between gravel augmentation, fluvial processes and form, habitat for aquatic animals of special concern, and population dynamics? What are the potential long-term negative impacts of gravel augmentation on biological, physical and chemical processes? How far can rivers be scaled down and still retain critical habitat functions? What impact will climate change have on stream restoration projects in California? Is gravel augmentation sustainable?