POTENTIAL EFFECTS OF SEDIMENT EROSION ON CHUM SALMON REDDS IN THE GRAYS RIVER, WASHINGTON

Riverbed scour can negatively impact buried salmon eggs, especially during strong flood events. An analysis of predicted scour depth was conducted so that it could be compared with known chum spawning areas and depths in the Grays River, Southwest Washington. Field data and hydraulic models were analyzed for several variables used in calculating scour depth. The resulting model predicted that only 2.6% of the Grays River watershed should be scoured at the 90th percentile flows. The maximum scour depth estimated by the calculations was 49.6 mm. This is not deep enough to affect chum salmon eggs that are usually buried at depths of 150-350 mm. Predicted scour locations were also compared with known spawning locations and scour did not occur in the same areas as chum spawning. Thus, scour during the 90th percentile flows in the Grays River should not have any impact on chum salmon eggs.