/ Knowledge of the three-dimensional connectivity between rivers and groundwater within the hyporheic zone can be used to improve the definition of fall chinook salmon (Oncorhynchus tshawytscha) spawning habitat. Information exists on the microhabitat characteristics that define suitable salmon spawning habitat. However, traditional spawning habitat models that use these characteristics to predict available spawning habitat are restricted because they can not account for the heterogeneous nature of rivers. We present a conceptual spawning habitat model for fall chinook salmon that describes how geomorphic features of river channels create hydraulic processes, including hyporheic flows, that influence where salmon spawn in unconstrained reaches of large mainstem alluvial rivers. Two case studies based on empirical data from fall chinook salmon spawning areas in the Hanford Reach of the Columbia River are presented to illustrate important aspects of our conceptual model. We suggest that traditional habitat models and our conceptual model be combined to predict the limits of suitable fall chinook salmon spawning habitat. This approach can incorporate quantitative measures of river channel morphology, including general descriptors of geomorphic features at different spatial scales, in order to understand the processes influencing redd site selection and spawning habitat use. This information is needed in order to protect existing salmon spawning habitat in large rivers, as well as to recover habitat already lost.KEY WORDS: Hyporheic zone; Geomorphology; Spawning habitat; Large rivers; Fall chinook salmon; Habitat management

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Understanding the factors that injure or kill turbine-passed fish is important to the operation and design of the turbines. Motion-tracking analysis was performed on high-speed, high-resolution digital videos of juvenile salmonids exposed to a laboratory-generated shear environment to isolate injury mechanisms. Hatchery-reared fall chinook salmon (Oncorhynchus tshawytscha, 93–128 mm in length) were introduced into a submerged, 6.35-cm-diameter water jet at velocities ranging from 12.2 to 19.8 m·s–1, with a reference control group released at 3 m·s–1. Injuries typical of turbine-passed fish were observed and recorded. Three-dimensional trajectories were generated for four locations on each fish released. Time series of velocity, acceleration, force, jerk, and bending angle were computed from the three-dimensional trajectories. The onset of minor, major, and fatal injuries occurred at nozzle velocities of 12.2, 13.7, and 16.8 m·s–1, respectively. Opercle injuries occurred at 12.2 m·s–1 nozzle velocity, while eye injuries, bruising, and loss of equilibrium were common at velocities of 16.8 m·s–1 and above. Of the computed dynamic parameters, acceleration showed the strongest predictive power for eye and opercle injuries and overall injury level, and it may provide the best potential link between laboratory studies of fish injury, field studies designed to collect similar data in situ, and numerical modeling.

Juvenile rainbow trout Oncorhynchus mykiss and steelhead (anadromous rainbow trout), fall (age‐0 and age‐1) and spring Chinook salmon O. tshawytscha, and American shad Alosa sapidissima were exposed to shear environments in the laboratory to establish injury–mortality thresholds based on estimates of strain rate. Fish were exposed to a submerged jet having exit velocities of 0 to 21.3 m/s, providing estimated exposure strain rates up to 1,185/s. Turbulence intensity in the area of the jet where fish were subjected to shear was minimal, varying from 3% to 6% of the estimated exposure strain rate. Injuries and mortalities increased for all species of fish at strain rates greater than 495/s. American shad were the most susceptible to injury after being subjected headfirst to a shear environment, while steelhead and rainbow trout were the most resistant. There was no apparent size‐related trend in susceptibility to high shear except that age‐0 fall Chinook salmon were more resistant to shear environments than age‐1 fall Chinook salmon. All groups of test fish exposed headfirst to high‐shear environments had higher injury–mortality rates than fish introduced tailfirst at similar strain rates. These results document the relationship between fish injury and a fluid force present at hydroelectric facilities and provide biological specifications for improving fish passage and survival.

We improved our predictions of fall chinook salmon (Oncorhynchus tshawytscha) habitat use by analyzing spawning habitat at the spatial scale of redd clusters. Spatial point pattern analyses indicated that redd clusters in the Hanford Reach, Columbia River, were consistent in their location from 1994 to 1995. Redd densities were 16.1 and 8.9 redds·ha-1 in 1994 and 1995, respectively, and individual redds within clusters were usually less than 30 m apart. Pattern analysis also showed strong evidence that redds were uniformly distributed within the clusters where interredd distances ranged from 2 to 5 m. Redd clusters were found to occur predominantly where water velocity was between 1.4 and 2 m·s-1, water depth was 2-4 m, and lateral slope of the riverbed was less than 4%. This habitat use represented a narrower range of use than previously reported for adult fall chinook salmon. Logistic regression analysis determined that water velocity and lateral slope were the most significant predictors of redd cluster location over a range of river discharges. Overestimates of available spawning habitat lead to nonachievable goals for protecting and restoring critical salmonid habitat. Better predictions of spawning habitat may be possible if cluster-specific characteristics are used.