I have had luck repairing a leak in my dam using the blanket method explained in this article.
http://www.aces.edu/dept/fisheries/rec_fishing/documents/constructionKentucky.pdfYou will have to pump your pond down till you do not leak any water, so finding where the water is comming out and watching for it to stop is key. My leak was only about 1/2 way down my dam, there for I did not have to drain the whole pond. When you see it stop leaking pump down another foot. Good luck its a royal pain to do. Took 6 buddys and alot of beer to get all the rocks and debris off the inside of the dam. Then theres getting your tiller on the steep bank, I tied it off to my fourwheeler as I went. Saved it from going in the drink a cpl times. Then used a regular gas tamper with 3inch 4x4's. works best doing the whole dam in 6'x6' sections at a time.
