The Chesapeake and Ohio Canal, abbreviated as the C&O Canal, and occasionally referred to as the "Grand Old Ditch," operated from 1836 until 1924 parallel to the Potomac River in Maryland from Cumberland, Maryland to Washington, D.C. The total length of the canal is about 184.5 miles (300 km). The elevation change of 605 ft (185 m) was accommodated with 74 canal locks. To enable the canal to cross relatively small streams, over 150 culverts were built. The crossing of major streams required the construction of 11 aqueducts (10 of which remain). The canal also extends through the 3120 ft (950 m) Paw Paw Tunnel. The principal cargo was coal from the Allegheny Mountains. The canal way is now maintained as a park, with a linear trail following the old towpath, the Chesapeake and Ohio Canal National Historical Park.