Hey Doug,
Just a few quick comments.
1) You should get a book called "The Earth Pond Sourcebook" by Tim Mattson. It deals extensively with the subject of site selection and everything else pond-related.
2) My existing pond site sounds very much like your potential site. There is a small swampy area upstream of my pond and a runoff creek from it feeds my pond. The creek dries up in the warm/rainless summer days, but during the spring it remains quite full. I do have the advantage of red clay soil that was used to form the embankment over 50 years ago. I would recommend a natural spillway over your embankment and for you to leave as much of the "swamp" intact as possible. It will collect sediment that you do not want in your pond and will be a MIGHTY breeding force for frogs, insects, and nitrogen-fixing bog plants. All of these things are excellent for a wildlife pond. Try to pick a spot that will collect as much "end of the run" runoff as possible and dig. Put your best material (hardpan clay) along the embankment and pack, pack, pack. Nothing is for sure in pond building. If you feel like your over your head and you don't consult any professionals, you may have the problem of not being able to have water over your head in the future. Bad pun intended.
Here�s a picture of my pond:
