Wow Theo! It's like you snuck into my dream and improved upon it.
Last March I started making plans for a very similar grow-out pond, just a little smaller -- about a quarter acre. Five months ago I scheduled my vacation from work so I could start excavating as of this Friday.
It is staked off. About two thirds of the water (4 to 5 feet, avg.) would be in an excavated basin, while the top third would be held behind a dam extending only four feet above present ground level -- or about a foot above a natural spillway. I know I've got good clay down as deep as my backhoe will dig. Even in this drought, my test hole has good water in it. Lots of big trees to clear.
Unfortunately, or maybe fortunately, several other priorities have taken precedence. It will be real interesting to learn from your experience.
I'll think about you over the next several days while you are slaving in the hot mud, fighting mosquitoes and wood ticks, as I attempt to save some saltwater fish from drowning.
Good luck,
Ken G.
