In addition to the above I have caught them with sculpin sticking out of their throats.
Plus, I believe Modest Man has mentioned that largescale suckers are the largest fish population in the Willamette.
Personally, I think that crayfish make up most of a bass' diet in places where there are not schooling bait fish. Growing up in PA I used to check stomach contents of bass caught in ponds that had tons of panfish, golden shiners, regular minnow types and crayfish. If I recall correctly over 90% of the fish I caught that had food in their stomach had crayfish. Fish remains were only in about 20% (some had both).
On the Willamette, I often have quite a few puked up crayfish littering my kayak at the end of a day of fishing.