Pont-siphon de la Durèze

Pont-siphon de la Durèze is one of the most striking surviving fragments of the Roman Gier aqueduct, located in the commune of Genilac in the Loire department, near the hamlet of Leymieux. It's a spot for travelers who enjoy tracking down ancient engineering out in the open countryside, away from the usual tourist crowds.

What exactly is it?

The Gier aqueduct was built in the 2nd century AD to carry water to Lugdunum — Roman Lyon — across a distance of 86 kilometers. The terrain along the route varied enough that Roman engineers had to rely on some unusual technical solutions, including siphons: sections where water traveled under pressure through lead pipes, following the principle of communicating vessels rather than the gentle, continuous gravity slope typical of most aqueducts.

The Durèze siphon was built specifically to carry water across the valley of the Durèze stream. The bridge-siphon stretched nearly 700 meters, with the structure rising as high as 80 meters. Water flowed through nine parallel lead pipes — today, at the discharge reservoir (réservoir de chasse), you can still see seven of the original ten openings where those pipes departed.

What will you see on site?

The bridge itself, which once spanned the valley, hasn't survived — only a handful of isolated pillars remain scattered across the Durèze valley. The best-preserved and most visible element is the discharge reservoir, carved into the slope above the valley: the structure where pressurized water entered the siphon system. This is the section listed as a historic monument since 1962, and it's the main feature visitors come to see.

The Gier aqueduct as a whole was a massive undertaking — roughly thirty bridges, eight rows of arches, eleven tunnels, and four siphons, crossing the valleys of the Durèze, Bozançon, Mornantet, and Merdanson. The siphon at Genilac is one of the best-preserved and most legible examples of this technology along the entire route.

How do you get there?

The site sits near the hamlet of Leymieux, on the boundary between the communes of Genilac and Rive-de-Gier in the Loire department, at an altitude of around 250 meters, right where the Durèze stream meets the Gier river. The region is crossed by walking trails dedicated to the Roman aqueduct, so it's worth planning a longer hike or bike ride to take in other surviving fragments in nearby communes such as Chagnon or Saint-Chamond.

When should you visit?

Spring and autumn are the best times to come, when vegetation is less dense and temperatures are comfortable for longer walks. In summer, thick greenery can partly obscure some of the structure, while in winter the paths can get muddy after rain.

Frequently asked questions

Is there an entrance fee? No, the site is freely accessible. It sits in open countryside, with no ticket booths or entry restrictions like you'd find at a museum.

How much time should I set aside for a visit? Viewing the discharge reservoir itself takes about 30 to 45 minutes, but if you're planning a broader route following the Gier aqueduct, budget half a day or more.

Is it suitable for visiting with children? Yes, the terrain is mostly flat or gently sloped, though sturdy footwear is a good idea — the paths are unpaved and can get slippery after rain.

What's the difference between a siphon and a regular aqueduct? A classic aqueduct relies on gravity and a steady, gentle downward slope. A siphon becomes necessary when water has to cross a deep valley — it descends under pressure through sealed pipes to the valley floor and rises back up on the other side, using the principle of communicating vessels.