Hydraulic Changes Following Large Wood Introductions
Large wood (LW) addition is often part of fish habitat restoration projects. However, there is limited information about the spatial–temporal variability in hydraulic changes after LW additions. We investigated reach-scale hydraulic changes triggered after the addition of LW that are relevant to juvenile coho salmon survival. We used Nays2DH, an unsteady two-dimensional flow model, to quantify the patterns and magnitudes of changes in stream velocity and shear stress in three alluvial gravel reaches. The study sites are located in low gradient reaches draining 5 to 16 km2 in the Oregon Coast Range. Survivable habitat was characterized in terms of critical swim speed for juvenile coho and bed stability, considering the critical shear stress required to mobilize the median bed particle size. Model predictions indicated that survivable habitat during bankfull conditions, measured as the area with velocity below the critical swim speed for juvenile coho, increased by 95 %–113 % after the LW restoration. Bed stability also increased between 86 % and 128 % considering the shear stress required to mobilize the median bed particle size. Model predictions indicated more habitat created in the larger site; however, considering that wood would move more frequently in this site there appears to be a trade-off between the timing and the resilience of restoration benefits. Overall, this study quantifies how the addition of LW potentially changes stream hydraulics to provide a net benefit to juvenile salmonid habitat. Our findings are applicable to stream restoration efforts throughout the Pacific Northwest.
27. *Bair, R. T., Segura, C., & Lorion, C. M. (2019). Quantifying Restoration Success of Wood Introductions to Increase Coho Salmon Winter Habitat. Earth Surf. Dynam. Discuss., 2019, 1-19. https://doi.org/10.5194/esurf-7-841-2019
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The short- and long-term effects of large wood (LW) additions on overwintering habitat quality for juvenile coho salmon in three small (2nd and 3rd order) streams in the Pacific Northwest were also evaluated. Understanding the persistence of overwinter habitat is key to evaluating the persistence of LW services in creating fish refuge during high winter flows. Using a 2D hydraulic model, changes in habitat quality and availability during bankfull flow conditions were assessed 1- and 6-years after LW addition. Before the LW addition, poor overwinter conditions dominated all sites across low velocity, slow/deep flow, and stable sediment metrics, except for low velocity and slow/deep habitat conditions in Site 2. One-year after LW addition, suitable habitat conditions peaked across all metrics for all sites. The greatest increase in suitable habitat coverage one year after LW addition was consistently observed in the largest reach. By year six, suitable habitat conditions had declined from the initial peak but remained improved relative to pre‑restoration conditions. Persistence was greatest in sites with the poorest initial overwinter conditions, indicating that the pre‑restoration hydraulic state influenced the potential for lasting suitable habitat services. Despite modest declines from peak values, persistent increases in suitable and heterogeneous habitat conditions were observed across all sites. Long‑term changes in suitable habitat and hydraulic heterogeneity were effectively tracked by Gamma distributions, which documented systematic shifts in hydraulic variability after restoration. Overall, these results suggest that LW additions can increase suitable overwinter habitat while maintaining heterogeneous conditions up to six years after restoration.
Maffia, M.M., Segura, C., Lorion, C., and Warren, D.R. In preparation. Longevity of Large Wood Restoration Hydraulic Services to Improve Coho Salmon Habitat: A 2D Modeling Approach. In preparation for Water Resources Research.
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