If your problem was seepage, I would say that you stand a good chance of stopping the leak by adding to the dam and making it wider with high clay content soil. I stopped seepage in my small pond dam by adding more and more pipe and dirt until it sealed. Even though I cored the dam, it seeped due to allot of tree roots in the vicinity of the dam.
With what you are describing though, the only way to fix it would be to stop the water flow i.e. pump it out, breach the dam and start over. Or, wait for a drought.
VA
