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Guard Lock

The Big Slackwater Guard Lock, constructed of blue-gray limestone, permitted canal boats to enter and exit the Potomac River, traveling from canal prism to slackwater navigation. At this particular location, the canal prism was below river level and the guard lock was also used to raise the boats, thus allowing them to pass into the river.

Milepost 85.8

Guard Lock
Photo Credit: C&O Canal National Historical Park