Guard Lock 4 is located at Mile 85.8 on the Chesapeake and Ohio Canal towpath. It cannot be accessed by vehicle, so reaching it requires hiking or biking 1.2 miles (one way) upstream from the closest parking area, Dam 4. See the Locks and Lockhouses web page for an interactive location map.
Guard Lock 4 marks the downstream end of the Big Slackwater section of the C&O Canal. Because it would have been near impossible to construct a canal through the limestone cliffs that hugged the Potomac River from Guard Lock 4 to Lock 41 at Mile 88.9, the C&O Canal engineers decided to build Dam 4 so that 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.
It is at Guard Lock 4, a mile upriver from Dam 4, that the boats entered and exited the canal. Today the lock is plugged with concrete to keep the Potomac River from rushing inland and flooding the area.
All of the water from Guard Lock 4 down to Dam 3 at Harpers Ferry was let into the canal at this point. Just fifty yards or so upriver was an inlet weir where water was diverted from the Potomac River into a small holding pond. A control gate at the downstream end of the pond could be opened and closed to regulate the flow of water into the canal. The concrete control gate is still intact, though it is off in the woods. With ticks being a problem, it is best to view it from the towpath.
The inlet weir, which is basically a culvert under the towpath, is also intact. Like the guard lock, it has been plugged to keep water out of the canal area.
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.
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Last updated on June 6, 2024