††††A gentle gradient had to be maintained for the system to work. Aqueducts were necessary to carry these channels across valleys, since the flow of the water would otherwise have been interrupted.
Later, aqueducts were also used to carry canals, allowing cities to be supplied with goods by water transport.
Roman aqueducts were built in layers using rows of arches. This type of structure proved to be so strong that some, like the Pont du Gard in France, are still standing today.
