An aqueduct may be an open or enclosed canal, a tunnel, or a pipeline; an aqueduct bridge is a structure that carries a canal over a valley or a river. .
Aqueducts were constructed by ancient cultures, but the aqueduct system constructed by the ancient Romans was probably the most expensive in the ancient world. The first to be built by the Romans, Aqua Appia, was an underground aqueduct about 16 km long. It was built in around 310bc. The Aqua Marcian in Rome, about 90 km long was the first Roman aqueduct to carry water above ground Ancient Rome was supplied by more than ten aqueducts. Parts of several are still in use; they still supply water to fountains in Rome. The ancient Romans built aqueducts in all parts of their empire, many of which are still standing. .
In modern times, several aqueducts have been built in Europe. The Marseille aqueduct, completed in 1847, is 97 km long. The aqueduct supplying water to Glasgow, Scotland, is 56 km long. The Kaiser Franz Joseph aqueduct, in Vienna, is 322 km long. Most of it is underground. The main aqueducts under New York City are between 61 and 183 m below sea level. .
The largest and most expensive aqueduct system in the world is that supplies water to the southern California. One major source of water is the Colorado River. Nowadays, a suitable lake still has to be found to supply a city with water but a specially built reservoir stores the water and a trough. Is no longer used the water is lead underground or the way flowing through conduits which are lined with concert. Water is now used in industry as well as washing and drinking and aqueducts there for have to be very large. As an industrial city grows in size the air and water become gradually dirtier and more unhealthy and lakes of pure water have to be used from long distances away. Examples of Great Britain are Glasgow, which gets its water from Loch Katrine; Birmingham for which channels and aqueducts come 73 miles from the Elan valley in Wales; and Manchester which is supplied by Lake Thirlmere, 96 miles away from Cumberland.