Dam 4 is one of the seven dams built along the Chesapeake and Ohio Canal to impound water and regulate fluctuating water levels. Described as the best dam on the river, the original rubble-filled crib version was built by Joseph Hollman between September 1832 and June 1835. After ongoing problems with flood damage, Dam 4 was rebuilt in masonry in the late 1850s. Dam 4 spans the Potomac River and impounds water in a 13-mile-long upriver pool known as “Big Slackwater.” It provided water for the canal between here and Dam 3 at Milepost 62.2 near Harper’s Ferry. The site of fighting during the Civil War, the structure sustained severe damage from floods in 1879 and 1937.