The image shows the waste weir adjacent (upstream) to Lock 21 (Swain’s Lock). Built in 1906, it represents the earliest known use of solid concrete structures on the Chesapeake & Ohio Canal. During the casting of the concrete, the inscription “D.C., 1906, Lock No. 21” was scratched into the wet material (visible in the photograph below) by an unknown hand. The waste weir functioned as a controlled overfall to allow excess water out of the canal and into the Potomac River. The flow of water through a four-foot cast iron pipe was controlled by wooden stop planks. While the Lock is no longer in use, the waste weir is still used to control water levels.