Fifty miles shy of Cumberland, Dam No. 6 marked the terminus of the Canal until the final section was completed in 1850. Guard Lock No. 6 has been filled with earth. Remains of the dam extend into the Potomac.

Originally, the Canal company thought it could profit from coal boated down from Cumberland and on to Washington via the Canal. Unfortunately, the B&O Railroad extended its line to Cumberland—which provided a more viable method of coal transport. Because the B&O did not have tracks into Washington, they agreed to bring coal from Cumberland and load it on Canal boats from a siding at Dam No. 6.

The arrangement was only marginally successful. No more than 6,000 tons a year were shipped via Canal boat between 1842-1850. The railroad also used the agreement against the Canal—using it as a reason why the final 50 miles of Canal was unnecessary to build.