I think whatever it is has to be the problem. It may have dug a den into the bank and hit a sandy strata that is draining the body of water. It may be a small pocket that will eventually saturate or?

I don't know anything about beavers breeding habits or times. Call the local Game Warden or someone to find out about when they do have babies to identify it. It just might be a muskrat. They are smaller than beavers but I doubt that it makes a lot of difference to you. If beavers, you should see plenty of evidence of felled trees. In any case, I would eliminate it/them.