aqueduct (ăkˈwədŭkt) [key] [Lat., = conveyor of water], channel or trough built to convey water, chiefly for providing a densely populated region with a supply of freshwater. The flow in aqueducts is ordinarily by means of gravity, although pumps are often used. Some aqueducts consist of tunnels cut through rock, while others are conduits made of some sturdy material. For example, the conduit may consist of steel pipe, concrete, wooden staves, sheet-metal flume, or any of these in combination, the flow being controlled by slide gate and needle valves. Aqueducts enable many cities in the United States to obtain water from a considerable distance. Los Angeles, for example, draws much of its water from the Owens River by means of an aqueduct more than 230 mi (370 km) long. Most of the supply for New York City is conducted through the Catskill Aqueduct and the Croton Aqueduct. The topography of the land influences the design of the aqueduct; usually part of the structure is above ground and part below. Where feasible, an aqueduct may generate hydroelectric power as a byproduct of its operation. Typical of such use is the aqueduct system for Springfield, Mass., which generates power at the foot of Cobble Mt. in addition to supplying the city with water. Aqueducts were employed from early times, probably first in Mesopotamia. Their construction reached a peak of skill in Roman times. Portions of some of the original Roman aqueducts are still standing.