Dam 4 on the C&O Canal in Chesapeake and Ohio Canal National Historical Park

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Dam 4 and a stop gate are located at Mile 84.6 on the C&O Canal towpath. There is vehicle parking at the site. See the Locks and Lockhouses web page for an interactive location map.

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The Potomac River between Dam 4 and Lock 41 (Mile 88.9) is known as the Big Slackwater. Because it would have been near impossible to construct a canal through the limestone cliffs that hugged the Potomac River in this area, the Chesapeake and Ohio Canal engineers decided to build Dam 4 so that the river 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. The excess water was also used to fill the canal from here down to Dam 3 at Harpers Ferry.

Illustration of how water was controlled at Dam 4 on the Chesapeake and Ohio Canal (click to enlarge)

Boats entered and exited the canal, passing to and from the river, at Guard Lock 4 (about a mile upstream from the dam) and at Lock 41.

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

The original Dam 4 on the Chesapeake and Ohio Canal was made from piles of rocks and wooden cribbing. However, after being damaged repeatedly by flooding, a permanent masonry dam was built in 1856. It is 20 feet tall and nearly 800 feet across and is still used by a 1913 hydroelectric plant on the West Virginia side of the river. The water below the dam on the Maryland side is a popular place for shore fishing.

Also located at the dam is what is known as a stop gate. Sitting atop a stone foundation and straddling the C&O Canal is what looks like a covered bridge. This is a winch house, a building used to store an actual winch and planks of wood that are just a little wider than the foundation. When the Potomac River is on the verge of flooding, a temporary dam, or stop gate, can be built by using the winch to lower the planks one by one through the floor and into groves that have been cut into both sides of the foundation. Some water is let through, but the majority is stopped from rushing downstream into the canal and causing damage. After the danger passes, the planks are hoisted back up into the building, thus allowing water to flow through the gate once more. Unfortunately, the winch house itself was a victim of a flood. The current house is a replica built by the National Park Service in the 1970s.

Winch House and Stop Gate at Mile 84.6 on the Chesapeake and Ohio Canal towpath

Slots in the stop gate foundation held stacked wooden planks to form a dam, Mile 84.6 on the Chesapeake and Ohio Canal towpath

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