There are many variables that influence time of the run return. Flow, temperatures, and sediment loading are just three of the factors. We know concentrations of predator (seals and sea lions) also have some influence on return timing but were not sure how much they delay movement between the mouth of the Columbia and Bonneville Dam.
You can add salmon to death and taxes as a sure thing. A few salmon will stray into streams outside their natal grounds, but the majority of the fish will find their way home once they enter the Snake River.