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ID: 2588

Gaeuman, D., K. De Juilio, and T. Buxton. 2023. Stream bed disturbance and risk of salmonid redds scour in the Trinity River, CA. Report for the Trinity River Restoration Program (TRRP). Yurok Tribe Fisheries Department, Weaverville, California. Available:

Stream bed disturbance during high-flow events is a normal consequence of a natural hydrologic regime. Bed disturbance can be beneficial to some aquatic organisms and some life stages of individual species but can also pose risks to other organisms and/or life stages. Among the benefits to salmonids, bed disturbance can shift macroinvertebrate species composition and abundances from that of large, long-lived species to smaller, shorter-lived ones that are more available as prey for juvenile salmonids. High flow events during the salmonid egg incubation season, however, also have the potential to scour the stream bed to depths that cause mortality of salmonid eggs or alevins. This study investigates the extent and magnitude of stream bed disturbance generated by winter flood events in the Trinity River between Lewiston Dam and the North Fork Trinity River to assess the potential for generating beneficial bed disturbance that stimulates trophic production and other ecosystem services, as well as the potential risk that deep bed scour might pose to salmonid redds. We evaluate bed mobility with dimensionless shear stresses (τ*) derived from modeled shear stresses and a map of estimated median bed surface grain sizes throughout the study area. Discharges considered range from flows that generate shear stresses near the threshold of coarse sediment entrainment in the river up to the maximum authorized flow release from Lewiston Dam coupled with spring 100-year tributary flood events. The full study area is divided into seven segments, each of which is defined by major tributary confluences and characterized by different hydrology. For each segment, the extent of the stream bed subject to beneficial bed disturbance at different discharge levels is quantified by the fraction of bed area where τ* exceed 0.03. Likewise, the risk that salmonid redds could be scoured to the depth of their egg pockets is assessed in terms of the fraction of mapped salmonid redds located where τ* exceed thresholds for significant bedload transport and full stream bed mobility (0.06). We find that the range of floods considered herein will disturb between 25% and 60% of the stream bed area, depending on stream segment, but that the risk of scouring salmonid redds is comparatively small. In most of the study area, redds tend to be at locations where shear stresses are larger than the average for the full stream bed at small discharges but increase relatively slowly or decrease as discharge increases. Redd locations are nearly twice as likely to exhibit shear stress reversals (decreasing shear stress with increasing discharge) than the general stream bed in six of the seven river segments. Only in segment 1, which includes a high concentration of redds in an artificial channel adjacent to the Lewiston Fish Hatchery, is the proportion of shear stress reversals at redd locations and over the full bed similar. Prior research in the Trinity River suggests that the probability of bed scour to depths sufficient to harm salmonid eggs is about 8% for τ* greater than 0.06 and about 3% for τ* between 0.04 and 0.06. Our results indicate that the maximum floods modeled will produce τ* greater than 0.06 at between 5% and 23% of all redd locations, whereas 0.045 is exceeded at between 15% and 40% of redd locations. Combined with probabilities of deep scour reported in the literature, these areal percentages yield an estimated risk of scour to individual redds during a maximum flood over the full study area of about 2.3%.

First Posted: 2023-01-13 20:00:46

Post Updated: 2023-03-23 17:25:17