Should note that some of those rivers are closed.The Yakima River is a good place, at multiple spots starting at Easton and going down all the way to Selah. There are guidebooks at fly shops that list access points. All the local Fly shops have the latest hatch information. This forum has a list of fly shops and resources on other pages. Accessed off the Home page.
Also the North Fork Stillaguamish, the Skagit and the Skykomish river have had good fishing in past years.
You may want to hire a guide to float the Yakima. That's fun. Every fly shop has referrals for guided float trips