Knowing a little about Oswego's geology, do you hit bedrock on the bottom of the pond? There is quite the outcropping of shallow sandstone around you, so be careful to not hit that layer unless you are in a low area where the water is coming up through it and not down into it. If the latter, you may need to clay-line the pond to prevent leakage.

A good idea is to pick up the book offered on this site "Perfect Pond, Want One?"

First things first; dig a few test holes near/around the pond to see what is really down there if you plan to expand and/or excavate. You will then get a good idea if that sandstone or perhaps limestone is closer to the surface than you think and that you have the right soils to make it worth the investment.

And the snakes? You will probably still have them after a clean-out, but they are likely being forced to your pond after their usual haunts have dried up.