The Allegheny River Lock and Dam No. 4 in Harrison Township, Allegheny County, Pennsylvania, is a lock that was built 1920-1927, and opened in 1927, 24.2 miles upsteam from the mouth of the river in Pittsburgh. The lock and dam were built by the U.S. Army Corps of Engineers as a part of an extensive system of locks and dams to improve navigation along the Allegheny River. It has a single lock and a fixed crest dam, keeping the river depth at least 9 feet in the pool above the dam. The amount of water that flows over the dam is not controlled locally, so it does not provide flood control, but it does help provide water to municipalities.
Traffic through the lock was about 1.8 million tons in 1998, including shipments of coal, petroleum, sand and gravel, ore, steel, chemicals, fertilizer, salt, flour, lime, and slag.