Graduate Thesis Or Dissertation

Modeling Large Wood Impacts on Stream Hydrodynamics and Juvenile Salmon Habitat

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  • Large wood (LW) pieces are recognized as an important habitat component for salmon freshwater habitat. As such, they are often used in stream habitat restoration practices despite a lack of knowledge about their impacts on spatial and temporal hydraulic characteristics relevant to fish habitat. In this thesis we present results on the impacts of LW on reach scale stream hydraulics. We use hydraulic models to identifying patterns and magnitudes of change in aspects of the flow field that are relevant to juvenile Coho Salmon after an addition of LW. The Nays2DH model was used because of its capacity to model unsteady flows around LW structures in-stream. Flow conditions after LW addition were modeled in three alluvial plane bed gravel reaches of Mill Creek in the Oregon Coast Range, a long term salmon life cycle study site. Study streams are small, with a bankfull (Q[subscript bf]) discharge between 2.2 and 8.7 m³/s and bankfull widths varying from 5.5 to 10.6 m. Survivable habitat was characterized for the flow field in terms of a) velocity (v) less than critical swim speed of juvenile Coho Salmon (v[subscript crit] = 0.5 m/s); b) and bed refuge estimated based on the likelihood of the movement of the reach median bed particle size (D₅₀). Spatial and temporal increases in wetted area were also determined. After the addition of LW, the area of acceptable habitat increased 17-26% in terms of velocity, 31-39% in terms of substrate stability, and 24-35% in terms of wetted area at Q[subscript bf]. Similar magnitudes of increase were observed over an entire storm hydrograph in each site. Patterns of change in the flow field showed deep pools of slower water forming upstream of LW jams which agreed with initial field observations of geomorphic change. Newly inundated flood plains and bars represented large portions of the additional habitat after the addition of LW. These areas had low velocity and shear stress values indicating that their contributions to additional habitat will be resilient to fluctuation even if acceptable habitat metrics were to be adjusted.
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