I am in agreement with you, you do not have a swamp now. But by definition, what you want to create could possibly be considered a wetland because it cannot drain anywhere over the ground(if that is truly the case because of topograhy?) That's why I keep asking the question, do you really have no place to drain off the excess water to? Is the only choice for the water to leave the area by means of soaking into the ground? And if it is soaking into the ground at that extreme of a rate, then perhaps the soils in that spot are not the best for a pond and will need to be sealed with further expense. This all means more research for you to do.
Perhaps the guy before you had a low spot(swampy) there and put some fill in to dry it up. That may have happened 50, or 100 years ago. As regulations get tighter, these choices we make to do changes to our properties will be tougher. Protect yourself now with proper records in case you want to make more changes later(dredging it out again, expanding, etc.).
The cycle of life on earth...all ponds will become swamps, marshes, etc, possibly back to the point of being dry ground again. Dirt and everything else always wants to go to the lowest places. It's just a matter of time.
