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I used to live in Portland many years ago, and there wasn't much caught in the Willamette in the area immediately around Portland (at least at that time), except for Spring Chinooks around March or April (mostly trolling or anchor fishing from boats with gear). Your best best might be the Sandy River which is only 10 to 20 miles away. There is a run of silver salmon around Sept-Nov, and seasonal runs of summer and winter steelhead. Also, the Deschutes River is about a 2 hour drive away from Portland and there is usually good summer and fall steelhead fishing there. You should check out the Welches Fly Shop website at flyfish@flyfishusa.com , or Google it, as they have lots of info on fly and Spey fishing, river reports, etc, as well as info on guided trips on the Sandy and the Deschutes.
Also there are some rivers near the coast (about 90 minutes away), such as the Trask where large Fall Chinooks up to 60 lbs or so used to be caught on fly tackle in October and November (I don't know about current conditions). (I caught 53 and 34 lb Chinooks in the Trask one day several years ago on red yarn and eggs, before I was into fly fishing). You might also find some info about the Trask or other coastal rivers by contacting or phoning Jim Teeny , the developer of Teeny fly lines, located in the Portland area (Gresham, OR). (see his website, http://www.jimteeny.com/index.html)