Welcome to the forum and good luck with your problem.
If it happens to be the valve or along the pipe that is the problem you could have a diver just go down and put a cap over the end of the pipe. That would eliminate all the pipe joint and valve as the potential problem unless the leak is at the first joint of pipe inside the dam and it is leaking around the outside of the pipe into the joint.
If you ever needed to drain the pond you would need to remove the cap though which could be problematic. But it should solve your leakage problem unless you have leakage around the pipe into a joint. And if you have that you have much bigger problems, depending on how far inside the dam structure the leak is. I'm not a dam engineer so those are just my layman's ideas. Take them for what they are worth.
You can find large caps, fittings and such on line by doing a search. The plumbing supply houses near me carry the large pipe but not the various joints, caps, plugs etc. But I have found them to easily be ordered on line and delivered by UPS.
Last edited by snrub; 11/11/17 08:13 AM.
