| Summary: | In the past, logjams have been removed due to concerns about flooding, erosion, and destruction of property. However, logjams have been found to have many ecosystem benefits, including generating pools for salmonid spawning, propagule retention sites, and increasing biodiversity. Due to these benefits, engineered logjams are being placed back into rivers, especially in the Pacific Northwest of the U.S., to promote salmonid populations. Riverine habitats are created by variations in substrate size, depth, velocity, turbulence, temperature, and cover. All species have preferences for certain conditions. Even within a species, the size and age can also impact preference. Specifically for salmon, juvenile salmon prefer shallow and low velocity regions while adult salmon prefer deep and coarse substrate. The presence of the logjams alters the bed morphology of the river and availability of habitats. To better understand how design choices for a logjam can impact habitats, important parameters include solid volume fraction (SVF) and channel width. Thus, these two variables were varied in 8 constructed logjams. The wake generated by each logjam was analyzed in a recirculating flume for velocity, turbulence, and fine sediment deposition. All logjams displayed increased velocity in the unobstructed side and decreased velocity downstream of the logjam side. Varying the solid volume fraction had little variation in stream-wise velocity and root mean square of the stream-wise velocity (proxy for turbulence) to the first order. Differences in the quarter and half width spanning logjams were seen as the half-width logjam had increased velocity in the unobstructed side, a recirculation region, and lower turbulence in the center of the logjam and unobstructed side. Deposition of fine sediment for the quarter-spanning logjams were not greater than that of the open bed case with the same conditions. Thus, the fine sediment deposition observed downstream of logjams are not correlated with the solid volume fraction. Indicating the deposition could be correlated with bed depth or depositing further downstream, which could not be studied. The logjam width had a greater impact on velocity and turbulence than variations in solid volume fraction. All the logjams provided variation in available habitats for riverine fish. Considering both fish preferences and wake effects from logjam characteristics will result in more effective river restoration projects.
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