Redd scour occurs at high river flows. As the river rises, the water gets deeper and moves faster. Depth and velocity directly affect redd scour. Pink salmon dig the shallowest redds in the smallest gravel, generally. If flows reach flood stage, which I think is 28' on the Skagit, pink salmon redds will experience over 98% egg mortality. The 2025 run will be reduced accordingly. Chum salmon also take a big hit from floods, but many of them have not yet spawned, so that helps. And chum often seek out side channels that shelter them from the brunt of high river flows.