Moat Feeder Canal
In wartime the moat was filled with water from the South Channel the Savannah River. This feeder canal featured stop-lock gates which provided water control and access to small barges bringing supplies to the fort. When filled from this ditch, the moat formed an effective barrier to hinder the approach of infantry.
Plaque courtesy Lat34North.com.
Original page, with additional info, here.
Photo credit: Byron Hooks of Lat34North.com.