An aqueduct is an artificial channel that is constructed to convey water from one location to another. The word is derived from the Latin aqua, "water," and ducere ("to lead"). The word is also used for any bridge that carries water, similar to viaducts, though they carry water instead of a road or railway. Sufficiently large aqueducts may also be usable by boats or ships. While a road bridge often carries the roadway at a more elevated level than the rest of the road, such a variation of height is not possible for an aqueduct.
Although famously associated with the Romans, aqueducts were devised much earlier in the Near East and Indian subcontinent, where peoples such as the Egyptians and Harappans built sophisticated irrigation systems. Roman-style aqueducts were used as early as the 7th century BC, when the Assyrians built a limestone aqueduct 30 feet (10 m) high and 900 feet (300 m) long to carry water across a valley to their capital city, Nineveh. The full length of the aqueduct ran for 50 miles (80 km).
Indian subcontinent was one of the earliest builders of aqueducts. More prominent evidence can be found at the sites of present day Hampi. The massive aqueducts near river Tungabhadra supplying irrigation water were once 15 miles long.The elegant water ways in royal center supplied water for royal bath houses.
Much of the expertise of the Roman engineers was lost in the Dark Ages, and in Europe the construction of aqueducts largely ceased until the High Middle Ages. An example of an extant small scale aqueduct system built in 1202 by Cistercian monks is the Spanish Real Monasterio de Nuestra Señora de Rueda, whose central heating and waste disposal system relied upon a series of aqueducts originating from a diversion of the Ebro River. Through most of the Middle Ages and even up to the 19th century, water was instead usually supplied through the digging of wells, though this could cause serious public health problems when local water supplies became contaminated. One notable exception was the New River, a man-made waterway in England, opened in 1613 to supply London with fresh drinking water over a distance of 38 miles (62 km). The development of canals provided another spur to aqueduct building.
The 19th century saw aqueduct building resume on a large scale to supply fast-growing cities and water-hungry industries. The developments of new materials (such as cast iron) and new technologies (such as steam power) enabled significant improvements to be made. For instance, cast iron permitted the construction of larger, more highly pressurised inverted siphons, while steam- and electrically powered pumps enabled a major increase in the quantity and speed of water flow. England led the world in aqueduct construction, with notable examples being built to convey water to Birmingham, Liverpool and Manchester.
In modern times the largest aqueducts of all have been built in the United States to supply that country's biggest cities. The Catskill Aqueduct carries water to New York over a distance of 120 miles (190 km), but it is dwarfed by aqueducts in the far west of the country, most notably the Colorado River Aqueduct, which supplies the Los Angeles area with water from the Colorado River nearly 250 miles (400 km) to the east, and the 444 mile (714.5 km) California Aqueduct which runs from the Sacramento Delta to Lake Perris.
Another widespread use for aqueducts is to supply large cities with clean drinking water. Some of the famed Roman aqueducts still supply water to Rome today. In California, USA, three large aqueducts supply water over hundreds of miles to the Los Angeles area. Two are from the Owens River area and a third is from the Colorado River.
In more recent times, aqueducts were used for transportation purposes to allow canal barges to cross ravines or valleys. During the Industrial Revolution of the 18th century, many aqueducts were constructed as part of the general boom in canal-building.
In modern civil engineering projects, detailed study and analysis of open channel flow is commonly required to support flood control, irrigation systems, and large water supply systems when an aqueduct rather than a pipeline is the preferred solution. The aqueduct is a simple way to get water to other ends of a field.
In the past, aqueducts often had channels made of earth or other porous materials. Significant amounts of water are lost through such unlined aqueducts. As water gets increasingly scarce, these canals are being lined with concrete, polymers or impermeable soil. In some cases, a new aqueduct is built alongside the old one because it cannot be shut down during construction.
See also: List of aqueducts in the Roman Empire
Navigable aqueducts are bridge structures which carry canals over other rivers, valleys or railways or roads. They are primarily distinguished by their size, carrying a larger cross-section of water than most water-supply aqueducts. Although Roman aqueducts were sometimes used for transport, aqueducts were not generally used until the 17th century when the problems of summit level canals had been solved and the modern canal system started to appear.
Early aqueducts such as the three on the Canal du Midi (1683) were stone or brick arches, the longest span being 18.3m on the Cesse Aqueduct (1686). However the weight of the construction to support the trough with the clay or other lining to make it waterproof made these structures clumsy and it was not until 1796 that the first large cast iron aqueduct was built at Longdon-on-Tern by Thomas Telford on the Shrewsbury Canal. It has a total length of 57m with 3 intermediate piers. Within 10 years he had completed the far more ambitious Pontcysyllte Aqueduct over the Dee valley on the Llangollen Canal which has a total length of 307m. Other cast iron aqueducts followed such as the single span Stanley Ferry Aqueduct on the Calder and Hebble Navigation in 1839 with its innovative 50m through arch design.
The impact of new materials can be seen in the experience of the Canal latéral à la Loire in France. It had 2 substantial arch aqueducts on the higher stretches of the Loire, the longest being 470m completed in 1838, but a river-level crossing was used to cross the Loire to the Canal de Briare because the consequent obstruction to the river during flooding was considered unacceptable. This proved troublesome until the 662m long steel Briare aqueduct was built in 1896, which remained the longest aqueduct in the world until the 21st century when the Magdeburg Water Bridge in Germany took the title.