The Willamette and the Columbia are both loaded with smallmouth. Fish rocky shore areas with a spinnerbait, minnow plug, crankbait, live bait etc. Especially look for areas with something that is a bit different than the surrounding area (bigger rocks, eddy, log, etc). Not as many largemouth up around Portland. I see reports of them being caught in Blue lake. Also, I sometimes catch them in backwaters on the Willamette. However, I probably catch a hundred smallmouth for every largemouth.
Best of luck!