I think that it's not smart to create a large pond without draining possibility. It's nice that you thought about that now but there is only one problem - the water. You won't be able to work in the water and that means that draining the pond (at least partly) is the only solution.

Here's an example:

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=356281&page=1