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Habitat quality of historic Snake River fall Chinook salmon spawning locations and implications for incubation survival. Part 2: intra‐gravel water quality

Reintroduction of fall Chinook salmon into the Swan Falls Reach of the Snake River is of regional interest; however, water quality has been considerably degraded. We examined the quality of the incubation environment within historic spawning sites by monitoring temperature, dissolved oxygen (DO) and the presence of hydrogen sulphide (H2S) within the water column, the undisturbed hyporheic zone, and simulated redds, during two annual periods representative of fall through spring incubation periods. The hydrographs of the two years differed, with year one representative of high flows and year two of low flows. The thermal regime during the two years also differed: year one was warmer overall than year two. Water column DO remained near saturation throughout both years. Within the undisturbed hyporheic, DO was lower than in the water column during both years, and quickly decreased to relatively anoxic conditions. During both years, the DO within simulated redds remained sufficiently high (> 7.0 mg l−1) to support fall Chinook incubation through the hatch phase of development. However, as each incubation season progressed, conditions in simulated redds decreased to less than 6.0 mg l−1 prior to the estimated beginning of emergence, and tended toward levels less than 4.0 mg l−1 prior to the estimated completion of emergence. Hydrogen sulphide was detected at highly toxic levels in the hyporheic zone and within some simulated redds during both years. Depressed DO conditions and the presence of H2S within simulated redds prior to, and throughout the period of estimated emergence suggest that survival of incubating salmon in the Swan Falls reach would be variable and low, and that relative survival would not be sufficient to support the reintroduction and sustainability of a viable population of fall Chinook salmon to that reach. Copyright © 2005 John Wiley & Sons, Ltd.