IAAL: I don't know about in GA, I am sure you are more familiar with the wide range of laws in other states, but in CA whatever the deed says is the boundry, is the boundry. So, if the deed says that the center of the creek is the boundry, and the creek moves, so does your lot line. My wife worked in the Title industry for years, and saw many cases of people loosing chunks of land because of descriptions like Ray's. She has even seen people loose chunks of land because they didn't build their fence right on the property line.
Again, this is CA, so GA may be different, but here in CA the lot line is exactly what the Grant/Waranty/Transfer deed says, even if it sound silly. It may be too late, but I would look into trying to work out a deal with your neighbor to redraw the lot line between your parcels to refernce a static land mark, such as the distance from the front of your lot, or a GPS value. Being that your neighbor has staked a ...er...stake in your lot, it may be too late for a friendly solution. It's worth a try though.
Either way, a local escrow company can tell give you an answer, as they deal with lot lines every day, and if you did get a lot line adjustment to a fixed reference, you would want to use an escrow company anyway.