I have thought about your suggestion and I belive that may work. Especially if you repack the dam with a mixture of clay and bentonite. I believe the rest of the barrel on the down stream side would have to be filled in with clay also. Our plan would be to clean out the pond while it is drained. What ever we use to do that will depend on how soft the bottom is so we won't get the equipment mired down.
Oldbob