Russ,

Why not just build up the dam with clay? If the add-on is wide enough at the base and tamped down, it will never leak. Adding good thick sod would help prevent erosion.

Would the add-on be the main spillway?

If so, you would have to guard against wash out. A thin layer of concrete, asphalt, etc would be necessary. Keeping the overflow wide and shallow as possible will help prevent washout.

Shrimp