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.
Most of the supply for New York City is conducted through the Catskill Aqueduct and the Croton Aqueduct . 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.