Water, via aqueducts, may be led from one place to another in several ways. Under the ground it is usually carried in pipes or in big tunnels. Over the ground it may be carried in small open channels or in larger ones called canals.
If the channel or canal has to cross a valley or hollow it may be carried on a bridge and it is this arrangement which is usually called an aqueduct; the word can also mean the system from source to destination. A large pipe or channel for water supply is often called a conduit.
From ancient times, cities have been built on the banks of rivers because it was easier to travel and to bring supplies of goods by water than by land. Before water was laid on in houses, the people of a city washed themselves and their clothes in the river and took their animals down to drink there. They also got rid of much of their refuse and sewage matter by throwing it into the river. As a result, the river water became dirty and unpleasant to drink, besides being so full of germs that it often made people ill. So it became necessary to find a special supply of unpolluted water. This is still a problem today.
One way of dealing with this problem is to build an aqueduct to connect the city with a source of pure water. Because the water in a channel always flows downhill, this source of pure water has to be higher than the city unless the water can be pumped.
Because the open countryside is not often smooth-and flat, without any hills or valleys, a simple channel to connect the source and the city is rarely possible. Instead, where it would be too expensive to lead the water round such natural obstacles, it is usual for the engineer to design a combination of pipes and tunnels, canals and aqueducts or siphons through them.
Aqueducts across valleys used to be built by erecting layers of arches one above the other until the top of the topmost layer reached the level of the channel at that point on its journey to the city. Modern aqueducts are built as boxes of concrete carried on piers.
Nowadays engineers rarely build aqueducts on bridges across valleys. More often, because they now have pipes that can stand high water pressure without bursting, they run pipes down one side of the valley and up to the same level on the other side. This is called a siphon.
Aqueducts were built and used long ago in Palestine, Egypt and Greece, but the most famous ones of the ancient world were those that served Rome.
In 312 B.C. Appius Claudius Caecus built a great road called the Via Appia (Appian Way), and a great conduit, the Aqua Appia (Aqueduct of Appius). The Aqua Appia was more than 10 miles long and ran underground all the way. Forty years later another underground conduit, the Anio Vetus, which led from the River Anio, was dug. It was more than 30 miles long. In the and century B.C. the first above-ground channel with real aqueducts leading to Rome was built. This was the Aqua Marcia, which was nearly 90 kilometers long.
Perhaps the finest and highest aqueduct bridge is the Pont du Card near Nimes in southern France. This was built in about 19 B.C. by the Roman general Agrippa to bring water from the Card River to Nimes. The bridge is 270 meters long and it carries a channel, covered by stone slabs, 120 centimeters broad and 130 centimeters deep.
Nowadays, a suitable source still has to be found to supply a city with water. A reservoir is often built at or near this source so that even in dry weather the aqueduct can be filled. The water is usually led underground to prevent it being contaminated as it flows through the countryside. It also avoids using valuable farming land.
Water is now used in industry as well as for washing and drinking, and aqueducts therefore have to be very large. As an industrial city grows in size the sources of water near it become gradually dirtier and more unhealthy, and sources of pure water have to be used from long distances away. Examples in Great Britain are : Glasgow, which gets its water from Loch Katrine; Birmingham, whose water comes 117 kilometers from the Elan valley in Wales; and Manchester, which is supplied from the Lake District, nearly 160 kilometers away.
Two much larger channels with aqueducts are the Colorado River aqueduct in the United States, which is 390 kilometers long and carries water across mountain ranges to serve several cities, and one 565 kilometers long which was built in 1903 to supply the gold-mining centers of Western Australia.