An aqueduct is an artificial channel built to transport water. It may be an open or enclosed canal, a tunnel, or a pipeline; an aqueduct bridge is a structure that carries a canal over a valley or a river. Water from a spring in the hills is collected in a reservoir to build up pressure and ensure a steady supply to the city. This water flows along the aqueduct (at a gradient of about 1 in 1000, meaning than for every 1000 feet in length, the water would flow downwards 1 foot) upon reaching the city, is is distributed via the many public fountains and pipes.