The Romans constructed aqueducts throughout their Republic and later Empire, to bring water from outside sources into cities and towns. Aqueduct water supplied public baths, latrines, fountains, and private households; it also supported mining operations, milling, farms, and gardens.
Aqueducts moved water through gravity alone, along a slight overall downward gradient within conduits of stone, brick, or concrete; the steeper the gradient, the faster the flow. Most conduits were buried beneath the ground and followed the contours of the terrain; obstructing peaks were circumvented or, less often, tunneled through. Where valleys or lowlands intervened, the conduit was carried on bridgework, or its contents fed into high-pressure lead, ceramic, or stone pipes and siphoned across.
Most aqueduct systems included sedimentation tanks, which helped to reduce any water-borne debris. Sluices, castella aquae (distribution tanks) and stopcocks regulated the supply to individual destinations, and fresh overflow water could be temporarily stored in cisterns. Public fountains took priority over the supply to public baths, and both took priority over supplies to wealthier, fee-paying private users. Some of the wealthiest citizens were given the right to a free supply, as a state honour. In cities and towns, the waste water from aqueducts watered gardens or scoured the drains and public sewers.
Rome's first aqueduct was built in 312 BC, and supplied a water fountain at the city's cattle market. By the 3rd century AD, the city had eleven aqueducts, sustaining a population of over a million in a water-extravagant economy; most of the water supplied the city's many public baths. Cities and towns throughout the Roman Empire emulated this model, and funded aqueducts as objects of public interest and civic pride, "an expensive yet necessary luxury to which all could, and did, aspire". Most Roman aqueducts proved reliable and durable; some were maintained into the early modern era, and a few are still partly in use. Methods of aqueduct surveying and construction are noted by Vitruvius in his work De architectura (1st century BC). The general Frontinus gives more detail in his official report on the problems, uses and abuses of Imperial Rome's public water supply. Notable examples of aqueduct architecture include the supporting piers of the Aqueduct of Segovia, and the aqueduct-fed cisterns of Constantinople.