face 72 --i agree with dave from eldorado i built what sounds very similar to your idea this past oct/nov. i had a hill coming down into a bottom that had a creek in it ---i didn't want to dam the creek so i created a long dam--from one point on the hill around the bottom area and tied it back into the other side of the hil---following the natural contour of the hill--it was necessary to core the dam all the way around and it was 3 ft to 6ft in places (thru top soil then sand that would have leaked pretty bad) before we hit good clay material to tie into and i'm in the middle of "clay" country. it's an expensive way to build a pond but i like mine. if i were digging it or having it dug i would have a reliable contractor in the area come out and dig some test holes along the "dam" line to see what you have to work with--and how far down you'd have to dig down to have good soil/clay to tie your core into.