Did the FBI dredge the river after the money find?
Not that I am aware of. Why would they? I am actually surprised they didn't explore Vancouver Lake ... like FlyJack suggests might have been a good idea. Did the FBI even explore the Shillapoo? Did they talk to locals and hobos?
The FBI was relying on their flight path and jump zone given by "smart guys". Then they were relying on "smart guys" Palmer etc. The FBI was looking/waiting on any report of a body being found ,,, checking hospitals etc. I think the FBI thought the Ingram money 'could not have any direct connection' with the flight path or where Cooper bailed and beyond that it was like looking for a needle in a haystack. The FBI doesn't look for needles in haystacks or indirect causal connections unless there is very good reason to do so. The FBI was waiting for forensic work to point to a solid connection ... and then would search.
Every solid connection or lead the FBI felt might produce fruit, they jumped on and explored. The Tacoma lead, the Eugene lead brought up by R2 etal, .. if they had had any reason to believe that Cooper might have angered into Vancouver Lake or the Shillapoo they would have been all over that with the same manpower they used at Ariel!
Why didn't they launch a massive search of the Washougal in 1980!
Because there was nothing but rumor and speculation to go on.