First off I'm in central TN. I had a run off pond dug 20+ years ago. The pond is about 200 ft across and in kind of a small valley with a dam at the deep part. At the time the guy who dug it said it might not hold water and he was partially right. It fills during the winter, stays full and drains in the spring. It used to pick a day and drain like someone pulled the plug on a bathtub, then stay dry all summer. There isn't much dirt at the shallow end and there are two large rocks showing. Average soil depth here is 3ft. Since I retired I have taken on the pond as a challenge. I first thought the dam was leaking so I added 60 yards of soil in the bottom at the base of the dam over two years. Now the pond fills partially during the year but always leaks slowly down. It usually fills to the exposed rocks in the shallow end which I took as a sign there wasn't enough/any dirt over the rocks. One possibility. The other thing I noticed was that on the back side of the dam sumac is growing, not the poison variety, but only halfway down from one end and top to bottom. Another sign? Back side of the dam isn't soggy.

I'm looking for suggestions as to the possible causes of the leaking