Hey Andy,
I've fished the Cowlitz from the Barrier Dam to I-5 with my 12ft Valco and a 25HP Johnson with a jet. My boat is rated for 15 hp and I get about 15 hp at the pump. It works fine, but I would not want any less power going up Blue Cr corner, or any of the fast water above the trout hatchery. I think if you stay down by the Toutle, with a small prop job, you'll be OK. I haven't fished down below the Toutle since the mountain blew, but the fall before, we hammered the kings and silvers, freedrifting and tossing golf tee spinners.
Mitch