more about aqueducts

Ancient Rome had eleven major aqueducts, built between 312 B.C. (AquaAppia) and 226 A.D. (Aqua Alexandrina); the longest (Anio Novus) was 59 miles long. It has been calculated that in imperial times, when the city's population was well over a million, the distribution system was able to provide over one cubic meter of water per day for each inhabitant: more than we are accustomed to use nowadays. 

In order to keep the gradient constant, the aqueducts took a roundabout route, following the contours of the land and heading along spurs which led towards Rome. As time went on, Roman engineers became more daring in the construction of high arches to support the conduits across valleys and plains and some of the later aqueducts were as much as 27 meters (about 100 feet) above ground level in places. Closed pipes were occasionally used to cross valleys by the "inverted syphon" method: the pressure forced the water down and up again on the other side, to a level slightly lower than before. But this system was costly, as it required lead pipes (lead had to be imported from Spain or Great Britain) and it was difficult to make joints strong enough to withstand the pressure; so arches were far more common.

Except where closed pipes were used, the channel in which the water flowed was just over three feet wide and about six feet high, to allow workers to walk throughout its length - when the water supply had been cut off - for inspection and maintenance. Where the aqueduct went through impermeable rock it was not lined, but where the rock was porous, and where the conduit ran on arches, a layer of impermeable concrete was applied to form a waterproof lining (opus signinum).

Every now and then there was a sedimentation tank, where the flow of water slowed down and impurities were deposited. Where two or more conduits ran near one another, as was common in later times, there were places in which water could be exchanged between them, either to increase the flow of an aqueduct carrying little water or so that one of the conduits could be emptied for maintenance and repair.When the water reached Rome, it flowed into huge cisterns (castella), situated on high ground, from which it was distributed through lead pipes to the different areas of the city. Part of the water was for the emperor's use, part of it was sold to rich citizens, who - for a price - could have it piped to their private villas, but much of it was available to the populace through a network of public fountains, which were located at crossroads throughout the city, never more than 100 meters apart. Enormous amounts of water went to supply the numerous baths complexes, such as the Baths of Caracalla.

For centuries, an army of laborers was constantly at work, under the supervision of the curator aquarum, extending and repairing the water system. But in the 6th century A.D., as the power of the Empire began to decline, the Goths besieged Rome and cut almost all the aqueducts leading into the city. (The only one that continued to function was that of theAqua Virgo, which ran entirely underground.) One or two were later restored and were used during the Middle Ages, but most of the population had to resort to the Tiber as the only source of water: it is for this reason that the medieval buildings of Rome lie almost exclusively in the two great bends of the river, the Campo Marzio and Trastevere. It was not until Renaissance times that the Eternal City was once again provided with aqueducts and fountains.