When the pond leaks down about 30-36 inches from full, the water stops coming under the dam, so the pond wouldn't have to be drained. The "bowl" or excavated part is about 8 feet deep. I don't know if it's roots left in the dam, or lack of an original core trench. A trench dug on the back side of the dam would let me discover the reason for the leak. As you have probably already read, the pond was renovated in August 2015, but about 2/3 of the dam was left in place. There were many trees on the dam between 6-14 inches diameter; ash, basswood, willow, elm, and cedar. The root balls were pushed out, but probably many side roots were left behind. The inside wall of the dam was scraped clean and left. If it's dead roots, there probably is only a few inches of clay covering them on the inside face of the dam. I need to do something about it this year. I'm tired of pumping it up several inches in a day, only to have it leak back down in about nine days.