Can you put up a coffer dam in any of the areas that feed that pond, rent a larger pump and dewater the pond quickly? Then get the work done asap and pull the coffer dam out.
What are the chances of the water rushing thru the area where the bad drain pipe is, eroding the area and loosing a lot of the water that is in the ponds?
Disturbing the beaver dam will tell you in one night whether they are still there or not. They will come that night to fix it if they are still there.
