Funny you mention that, when I was describing it to a friend I said yea it was the exact same size as that little coho hen you caught last year!
Those fish get big in the Tualatin - if you go to the north end of Jackson Bottom Wetlands (connects to the Tualatin during times of high water) right now, there's about 20 big ones trapped out in the last vestiges of the north end lake there wallowing around, kind of cool...a little depressing.
But yea, the river is small there at Rood Bridge, and very low right now, so I was a little surprised to see any there bigger than the trout-ish size ones I usually run into, let alone that big. I did notice the action disappeared in each hole after catching a couple, so not much room in there for too many of them I guess.