I've looked at all the GIS photo's I could find and it always seems to stay at the same level. There's even a small pit toward the back of the property that is connected by a drain and valve. Looks like they may have used it to pump water into when they were digging and them released it latter on when they finished.
The creek is fairly close, but I believe there is too much elevation between them.
Here's a picture. http://i1296.photobucket.com/albums/ag9/tmorgan28/Lake_zps205011a4.jpg