Visitors to the C&O Canal NHP today can see many structures which were integral to the operation of the canal, including locks, lockhouses, aqueducts, bridges, culverts, feeder dams, and waste weirs. Even the canal is a historic structure - the canal itself is called the "prism," because the surface of the canal bed is wider than the bottom, making it prism-shaped. The prism was lined with clay to keep the water sealed inside.
Many of the structures which now remain are made of stone. The stonemasons were the most prominent craftsmen of the canal, painstakingly cutting blocks of stone to as small a tolerance as a half-inch. Today, the highway bridges that vault the river and the canal are formed of concrete, but this sort of construction did not develop until well after the canal was built. Concrete was not used on the canal until 1906, and is most commonly found as replacement work in waste weirs and lock pockets.