That's a heck of a lot of territory to be covering. Since I just moved up here and don't have a lot of experience I will try my best being an Oregon native.
My Dad said that the Deschutes is on fire right now, so if you head out towards Bend then hit the Deschutes, or even Fall River outside Sunriver. If you want to stay west of the mountains hit the Salmon or Sandy rivers outside of Welches and Sandy, respectively, on HWY 26.
I hope that this helps a little, but there is a ton of water to hit between Seattle and Bend.
Matt