gotta say for sheer numbers of fish no river in the world matches the Cowlitz.. However numbers of fish isn't the only measure in my book. There is also the scenery, responsivness of the fish, amount of open water.. and just the overall quality of the experience.
My favorite river has to be the North Umpqua but thats because of the company I have when I am there, and the ethic of the anglers who are there.
On the Deschutes your constantly getting low holed or swamped by jet boats or you flat out can't fine an unocupied run..
The north umpqua has ver responsive fish you never get low holed, it's beautiful and the restruant has sour cream roll ups and grandmas bread. ( insiders info ) The river is always fishable from June 1 - the end of november. although there is another river i like almost as much it just doesn't qualifiy as a great river like the Umpqua does.
Plus the Umpqua has a rich steelheading history behind it. you can look down at the rocks and wonder who made the cleat marks. Zane grey , Major Mott, Col. hayden, Billy Pate? or just some guy who was there before you