Robinson,
I'm guessing there wasn't enough suitable material close by to line the whole lake? Could there be enough to cover the most likely trouble spots?
If you are confident that the water you found below the dam is indeed the same water leaving your pond perhaps some dye placed at likely sources in the pond would help minimize the amount of patching you would have to do.
I remember suggestions of placing sealed plastic bags of dye in a pond at suspect areas & then puncturing the bags in an attempt to locate the leak.
I'm sure it's a heartbreak situation for you but it's also a great learning opportunity that you are to be commended for sharing here.
Wishing you some luck, sounds like you're due.
