The aqueduct is an engineered conduit that carries water across long distances from distant sources to a city, flowing by a carefully maintained gentle gravity gradient through channels, tunnels and elevated arcades. Long-distance water channels were built in Assyria around 700 BCE, and the form reached its peak with Rome's eleven aqueducts, which delivered vast quantities of water to fountains, baths and households. The technology let a city draw on springs and rivers far beyond its own site.
The aqueduct decoupled a city's size from its local water supply, allowing dense populations to live where the ground itself could not support them. It made the large classical city possible and remained the model for urban water supply for centuries.