Chesapeake and Ohio Canal National Historical Park | LOCK 41

Lock 41 at Mile 88.9 on the Chesapeake and Ohio Canal towpath

Lock 41 at Mile 88.9 on the Chesapeake and Ohio Canal towpath


Lock 41 is located at Mile 88.9 on the C&O Canal towpath. It cannot be accessed by vehicle, so reaching it requires hiking or biking .8 mile upstream from the closest parking area, McMahon’s Mill. See the Locks and Lockhouses web page for an interactive location map.


Lock 41 on the Chesapeake and Ohio Canal marks the upstream end of the Big Slackwater section of the canal. Because it would have been nearly impossible to construct a canal through the limestone cliffs that hugged the Potomac River from Guard Lock 4 at Mile 85.8 to Lock 41, the C&O Canal engineers decided to build Dam 4 so the Potomac could be backed up to create a deep pool that was navigable year-round (unless it froze during the winter). Canal boats could travel directly on the river, thus eliminating the construction of three miles of canal, a big savings for the canal company.

Diagram of the Big Slackwater section of the C&O Canal

Diagram of the Big Slackwater section of the C&O Canal

It is at Lock 41 that the boats entered and exited the canal. The canal was actually at a lower elevation than the river, so by using the lift lock boats exiting the canal were raised to river level and boats entering the canal were lowered to canal level.

Remnants of Lock 41’s lock gate, Chesapeake and Ohio Canal National Historical Park

Remnants of the Lock 41’s lock gate, Chesapeake and Ohio Canal National Historical Park

Despite the canal boats being on the river instead of the canal, mules still had to pull them because they couldn’t travel upriver due to the current—there were no motors back then. It was also difficult for the mules when traveling downriver because the boats could actually get ahead of them. The original towpath was just an old trail that ran along the Potomac at the base of the cliffs. Back in 1996, a flood destroyed the trail, so those hiking or biking the towpath had to make a detour along local roads, and nobody could claim to have traveled the entire C&O Canal towpath. However, in 2010 the National Park Service began construction on a new concrete towpath and completed the project in October 2012. Once again it is possible to hike or bike from Georgetown to Cumberland without leaving the canal.

Reconstructed towpath along the Big Slackwater section of the Chesapeake and Ohio Canal

Reconstructed towpath along the Big Slackwater section of the Chesapeake and Ohio Canal

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Last updated on June 20, 2024
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