We haven't ever done it but have been told that you could dump charcoal or sawdust and that those will plug it up. If the charcoal would work I don't know why some really good packing gravel like ab3 or something wouldn't work as long as the water would carry it to the right spots.
Call someone in your area with an excavating business. Many of them build ponds. They may be able to come and evaluate your dam and determine why it's leaking and offer suggestions on how to fix it.
When C has a pond leaking (12 to 15 acres), he"ll drain it and then take a tractor and run over the bottom of the pond again and again. That's something we've done all the years we have had fish ponds.
Pond Dam
