Ive spent a lot of time on the Cowlitz fishing and drifting and am actually moving to property that is right next to the river next week (exclusively from the Mayfield damn down to Toledo, and the upper lakes) I've done some jump shooting on the river and have shot a few ducks while floating down it fishing, but have never intentionally set up on it. I have seen group of 3-4 screaming up and down the river from time to time, but I'd say that you better have a really good dog and set up where there's not much current, because my concern would be retrieving what you shoot in a lot of areas that I'm familiar with. That being said, there are a number of side channels and back sloughs you could set up on and do pretty well I think IF you can manage to garner permission. that is really the challenge, because most of the land around the river is private or hard to access.