Bob,
we dropped 4 ft and my daughter found a hole in the shallow end. it went straight through the clay and down through the rock. It was almost 3 inches in diameter. Filled and packed the hole with 2 bags of concrete and surrounded the dry concrete the next day with more and then we got 4 inches of rain so the pond raised 2.5 feet in 3 days and is holding steady. Looking for a nice break from the rain to make sure it holds but so far so good. I flagged 8 locations a long the shore line and the 2 days before it rained the water moved 1/4 of an inch from the flags in the shallow end which could easily have been from evaporation since it windy and was in the mid 90s before the rain came . Really hoping we got lucky and this was the only hole and the pond will hold now. Never had any water at the base of the dam and I'm glad I didn't buy anything to try and seal something because I would have been way off