Pont du Gard. The Pont du Gard is an ancient Roman aqueduct bridge built in the first century AD to carry water over 50 km to the Roman colony of Nemausus. It crosses the Gardon River near the town of Vers-Pont-du-Gard in southern France. The Pont du Gard is the highest of all Roman aqueduct bridges, and one of the best preserved. It was added to UNESCO's list of World Heritage Sites in 1985 because of its historical importance. The bridge has three tiers of arches, stands 48.8 m high, and descends a mere 2.5 centimetres-a gradient of only 1 in 18,241-while the whole aqueduct descends in height by only 12.6 m over its entire length, indicative of the great precision that Roman engineers were able to achieve using simple technology. The aqueduct formerly carried an estimated 40,000 m 3 of water a day to the fountains, baths and homes of the citizens of Nîmes. It may have been in use as late as the 6th century, with some parts used for significantly longer, but a lack of maintenance after the 4th century led to clogging by mineral deposits and debris that eventually stopped the flow of water. After the Roman Empire collapsed and the aqueduct fell into disuse, the Pont du Gard remained largely intact due to the importance of its secondary function as a toll bridge. For centuries the local lords and bishops were responsible for its upkeep, in exchange for the right to levy tolls on travellers using it to cross the river. Over time, some of its stone blocks were looted, and serious damage was inflicted on it in the 17th century. It attracted increasing attention starting in the 18th century, and became an important tourist destination. It underwent a series of renovations between the 18th and 21st centuries, commissioned by the local authorities and the French state, which culminated in 2000 with the opening of a new visitor centre and the removal of traffic and buildings from the bridge and the area immediately around it. Today it is one of France's most popular tourist attractions, and has attracted the attention of a succession of literary and artistic visitors. The location of Nemausus was somewhat inconvenient when it came to providing a water supply. Plains lie to the city's south and east, where any sources of water would be at too low an altitude to be able to flow to the city, while the hills to the west made a water supply route too difficult from an engineering point of view. The only real alternative was to look to the north and in particular to the area around Ucetia, where there are natural springs. The Nîmes aqueduct was built to channel water from the springs of the Fontaine d'Eure near Uzès to the castellum divisorum in Nemausus. From there, it was distributed to fountains, baths and private homes around the city. The straight-line distance between the two is only about 20 km, but the aqueduct takes a winding route measuring around 50 km. This was necessary to circumvent the southernmost foothills of the Massif Central, known as the Garrigues de Nîmes. They are difficult to cross, as they are covered in dense vegetation and garrigue and indented by deep valleys. It was impractical for the Romans to attempt to tunnel through the hills, as it would have required a tunnel of between 8 and 10 kilometres, depending on the starting point. A roughly V-shaped course around the eastern end of the Garrigues de Nîmes was therefore the only practical way of transporting the water from the spring to the city. The Fontaine d'Eure, at 76 m above sea level, is only 17 m higher than the repartition basin in Nîmes, but this provided a sufficient gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city. The aqueduct's average gradient is only 1 in 3,000. It varies widely along its course, but is as little as 1 in 20,000 in some sections. The Pont du Gard itself descends 2.5 cm in 456 m, a gradient of 1 in 18,241. The average gradient between the start and end of the aqueduct is far shallower than was usual for Roman aqueducts-only about a tenth of the average gradient of some of the aqueducts in Rome. The reason for the disparity in gradients along the aqueduct's route is that a uniform gradient would have meant that the Pont du Gard would have been infeasibly high, given the limitations of the technology of the time. By varying the gradient along the route, the aqueduct's engineers were able to lower the height of the bridge by 6 metres to 48.77 metres above the river-still exceptionally high by Roman standards, but within acceptable limits. This height limit governed the profile and gradients of the entire aqueduct, but it came at the price of creating a sag in the middle of the aqueduct.