That looks to me like the square hole for the big square axle of a grist mill, and the center that you see is the sand bottom of the creek with a few inches of water over it. That sure looks exactly like a water wheel. That would be a fantastic piece if it was possible to get it on a raft and float it out. Several 1100 x 24 truck inner tubes, and some rope and a hand tire pump and you might get it out of there. Bring a buddy and a 10' long 2 x 4 or 2 to use as a lever, work it carefully under and lift. If you have a buddy, use 2 levers. Then when wedged up, slide an inflated tube under and tie several together, and try to float it out. Make it so it wont tip, like tie 3 or 4 tubes in a triangle or square and have this in the center. Maybe have the 2 x 4s between the tubes and the wheel so it doesnt pop them. Maybe bring a hand saw to cut them in 2 pieces just to do that.