Been coming up for 30 years, now have a place in Richland to fish and hunt from.
In the old days the salmon run seemed much longer starting pretty good in mid sept, now October is a sure thing and spotty before that. Some years a bit different than others. Coho and Steelies are now hotter than ever with good numbers of respectable fish. I would still suggest October as the best bet if you want to see any number of fish and experience a possible run in front of you while you stand there in awe.
For most of us its not numbers of fish anymore but the quality of the experience with a lot of releasing. If you are our kind of fisherman it wont take much to please you so late Sept if the weather cools and the river temps go down with a decent flow of water.
Good luck and remember take a picture and- letem go.