Mill City Bridge is a very nice hole, my buddy caught a 26lb clipped chinook earlier this year from under the railroad bridge.
Another good spot is in (or pretty close to) Gates, I am unsure of the exact location (a friend showed me this spot), and it is a spot where you have to park and walk down this dirt hill that is a little steep at the beginning, then you arrive to an area on the Santiam that is just a huge whirlpool and you would be standing on a rock formation which the river cuts through and also flows around the other side, it would not be a pretty place to fall into. I haven't got around to getting my salmon/steelhead tag this year so I never flipped any gear out for bigger fish in this spot, I would like to know if anyone else knows what spot im talking about.