Lock 12 is constructed of dark granite sourced from a quarry near Glen Echo, MD, with coping of dark red sandstone. It is constructed of similar materials to those used in Lock 13 (post #5). Although the two locks are less than 0.1 miles apart, their designs are significantly different. Based on a plan from 1828, Lock 12 was intended to be filled with water through three culvert openings within each lock wall. Debris often clogged the openings of the culverts in this design and, in circa 1870, a bypass flume was added to resolve this issue. Originally, swing gates were used on the upper and lower ends of Lock 12. In the 1870s, in order to reduce travel times of the boats passing through, a drop gate was installed in the upper end. This was a solid partition across the lock that was raised and lowered by operating a metal wheel gearing system, a variation on the swing gates that open in the center (of which an example can be seen in the photograph of Swain’s Lock).
The construction of a drop gate necessitated the rebuilding of the upper end of the lock and added 10 feet to the internal length of the lock, from 91 feet to 101 feet. In comparison to the original stone work, the rebuilding of the upper lock walls was poorly done: today, these walls regularly leak water.