Abstract
Abstract. 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 of 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.
Highlights
Large wood (LW) is a fundamental component of many temperate streams given its influence on flow resistance, stream morphology, sediment transport, nutrient cycling, and stream habitat (e.g., Triska and Cromack, 1980; Harmon et al, 1986; Montgomery et al, 1995; Kail, 2003)
The decrease flow area around the wood is consistent with the increase in the mean water surface elevation (WSE) slope between pre- and post-LW in all sites (Table 2)
The changes in the velocity and shear stress distributions occurred as the flow encroached into the floodplain, and stream margins have been associated with the creation of off-channel habitat for juvenile coho salmon in previous studies (Swales and Levings, 1989; Bell et al, 2001), no quantification of the actual changes in the flow field in terms of velocity or shear stress had been conducted before
Summary
Large wood (LW) is a fundamental component of many temperate streams given its influence on flow resistance, stream morphology, sediment transport, nutrient cycling, and stream habitat (e.g., Triska and Cromack, 1980; Harmon et al, 1986; Montgomery et al, 1995; Kail, 2003). LW structures increase heterogeneity in the flow field by promoting local scour and sediment retention, reducing average flow velocity, influencing bed texture (Buffington and Montgomery, 1999a), and promoting increased interaction of the flow with the floodplain (Beschta, 1979; Harmon et al, 1986; Lisle, 1986; Bisson et al, 1987; Wipfli et al, 2007; Seo et al, 2008). Prior to the recognition of the role of LW pieces in habitats, forest management operations allowed harvest-
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