I don't know anything about flyfishing, but there is good access to the Willamette from the I-5 bridge near the d street boat ramp on up to day island park (in Springfield). I've seen lot's of fly fishers catch trout and steelhead in that section wading. True, like bran said, there will be people floating the river in tubes, but they tend to come in clumps, they're friendly, and you can fish in between them easy. If you get there early, you'll have the place to yourself with the exception of one other flyfisherman targeting steel (last year anyway).
Good luck to you,