Big Slackwater is downstream from McMahon’s Mill. Canal boats once entered the river here as the mules continued walking along the towpath. C&O Canal Company engineers chose to utilize the pools behind Dam 4 and Dam 5 rather than dig a canal prism because of rocky cliffs along the river. In 1996, severe flooding caused extensive damage to the towpath, making this section of the historic canal impassible for over 15 years. In the summer of 2010, the National Park Service began reconstructing the missing section of towpath and it was completed in the fall of 2012. Now, the “towpath” is a concrete pathway that hugs the river.