honestly, I have caught Coho in late August. But generally sept 1st is when I start fishing the vedder. However I live 1300 km away and am now stuck fishing the Kitimat, Skeena, Lakelse, Copper, Kalum you probably get the point.
From Sept 1st and on, get up early, fish with good roe at first light and if your in the right place, and dont let the theory that the fish are only in the lower river fool you. There are always fair numbers of Coho and tons of spring jacks and water conditions dont matter too much
I have very good days when the water was piss low.