Big Slackwater Restoration
Mile 86.7 - This is what the "missing section" of the canal looked like before the Big Slackwater restoration: a stop sign and a warning to go back and take the detour at Dam Number 4 Road, a dangerous route along twisty and narrow local roads.
This is what it looks like since the restoration (completed October 2012.) The towpath has been recreated by running a cement boardwalk along 2.6 miles of the cliffs that line the Potomac. The Park Service put up a sign which explains it all pretty well:
There is no canal here. Canal boats entered the river through special locks, and mules pulled boats from the towpath that hugged the often rugged shoreline. Canal engineers knew it would be almost impossible to blast a canal 60 feet wide and six feet deep into the limestone cliffs. The solution was to build Dam Number 4 downstream to create navigable water deep anough for canal boats being pulled along the river bank. They still had to blast rock--but only for a narrow towpath
This section of the river is called Big Slackwater. "Big" because it forms a resevoir over twelve miles long behind Dam No. 4, much longer than the half mile resevoir behind Dam No. 5. It's "slack" because this pool is normally more calm than other sections of the river marked by swift currents and protruding rocks.