The Pont du Gard crosses the Gardon river in the commune of Vers-Pont-du-Gard, in the Gard department of southern France, a short distance from the small town of Remoulins and some twenty-five kilometres north-east of Nîmes. It is not a bridge in the ordinary sense. What survives is the centrepiece of a Roman aqueduct, and the arches were built not to carry people or carts but to hold a covered water channel level as it crossed a river gorge. Three tiers of arches lift that channel 48.8 metres above the water, which makes the Pont du Gard the tallest aqueduct bridge the Romans ever raised anywhere in their empire. The lower tier carries six arches over 142 metres, the middle tier eleven arches over 242 metres, and the top tier originally held forty-seven small arches along some 360 metres, of which thirty-five remain across a length of about 275 metres. The structure tapers as it rises, from roughly nine metres thick at the base to three metres at the summit, and the widest of the lower arches spans 24.52 metres, set deliberately over the main channel of the river.
The bridge only makes sense as part of the system it belonged to. The aqueduct ran a little over fifty kilometres from the Fontaine d'Eure, a spring near Uzès, to the distribution basin at Nemausus, the Roman city that became Nîmes. The direct distance between the two points is barely twenty kilometres; the extra thirty were spent contouring around the hills of the garrigue because the engineers had almost no height to spend. The total fall from source to city was about twelve metres, an average gradient of roughly one in three thousand, and across the Pont du Gard itself the channel drops just 2.5 centimetres over 456 metres. Holding that tolerance over fifty kilometres of limestone country, with the surveying instruments available in the first century, remains the most impressive thing about the monument, and it is invisible to anyone standing beneath it. Water took close to twenty-seven hours to travel from the spring to the city, and the aqueduct is estimated to have delivered in the order of 200,000 cubic metres a day to the fountains, public baths and private houses of Nîmes.
