An aqueduct is an artificial channel that is constructed to convey water from one location to another. The word is derived from the Latin aqua, "water," and ducere, "to lead." The word is also used for any bridge that carries water, similar to viaducts, though they carry water instead of a road or railway. Sufficiently large aqueducts may also be usable by boats or ships. While a road bridge often carries the roadway at a more elevated level than the rest of the road, such a variation of height is not possible for an aqueduct.