I have heard a good amount of chatter on Marine 13 (I think they do bridge coordination there)
The USCG does safety announcements on 16, and makes the actual announcement on 22a.
(on 16 they will say "This is USCG Portland with information regarding Columbia river near mile marker 16, go to channel 22 alpha" -- Then they will go to 22a to make the announcement)
Those are the 3 that I have heard activity on, but remember, none of these channels have 'repeaters' so unless you are near the waterway, you may not hear a lot. (except 16, the USCG has multiple towers simulcasting)
I am fairly new to marine radio, so someone correct me if I am not quite correct.