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A canal lock or navigation lock is a device that lifts or lowers boats, barges or other vessels from one water level to another. Locks used on canals allow the negotiation of hills without recourse to lengthy detours, or the use of tunnels or aqueducts. The same kind of locks are used on rivers, often in connection with dams since there is generally a difference in water level between the upstream side of a dam and the downstream side.
A lock traditionally consists of two pairs of oak or elm gates placed one after the other along a navigable channel of water. Modern commercial locks consist of large steel gates but use essentially the same swinging gate design, with the exception of some low head locks that use sliding gates (see Kiel Canal) The system operates in much the same way as an airlock but acts between two levels of water as opposed to two levels of air pressure.