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Origin and history

The Gier Waterway, 86 km long, is one of four ancient aqueducts feeding Lugdunum (Lyon). Built in the second century, it draws its water from the Gier River, tributary of the Rhone, and has a elevation of 105 m with an average slope of 1.1 m/km. Its flow rate was estimated at 15,000 m3/day, thanks to various techniques: covered trenches, tunnels (including one of 825 m near Mornant), canal bridges, and four siphons to cross the valleys. Its remains, protected as early as 1875, include arches, tanks, and sections classified as Historical Monuments, such as the Beaunant Siphon Bridge or the arches of the Air Plat in Chaponost.

The date of its construction was debated. Originally attributed to Emperor Claude (I century) after the discovery of lead pipes stamped in his name, it was then associated with Hadrian (II century) thanks to the "stone of Chagnon" (1887), a stele prohibiting crops near the work to protect it. However, dendrochronological analyses in 2018 on wooden formwork of the Beaunant Siphon Bridge revealed a construction around 110, during the reign of Trajan, without excluding a completion under Hadrian. This combines august elements (Infrastructure Protection Act) and later advanced techniques.

The lake passes through 14 communes, including Soucieu-en-Jarrest, where emblematic remains such as the hunting reservoir of the siphon du Garon (classified in 1930) and the arches nicknamed "the camel". In Chaponost, the Air Plat presents 72 arches out of the 92 originals, with a reticulated cladding rare in France, while the 2.6 km long siphon of the Yzeron illustrates a technical feat with its 12 lead pipes resistant to 13 pressure bars. These sections, studied from the 16th century by scholars such as Guillaume Marie Delorme, were mapped precisely in the 19th century by Paul de Gasparin and Germain de Montauzan, whose 1908 thesis remains a reference.

The protection of the aqueduct rests on terminals such as the "Chagnon Stone" and the "Rieu Stone" (1996), repeating the prohibition of ploughing or planting nearby. These measures, inspired by Austrian laws, were designed to preserve underground pipelines, of which less than 100 (out of an estimated 1,000) were identified in 2001. Recent restorations, such as those of the Arches of Chaponost (2009-2010), have used reconstituted Roman materials. In 2018, the pipeline benefited from the Heritage Lotto, highlighting its historical and tourist importance.

The route of the water pipeline reveals architectural peculiarities, such as the redundant bypass of the Durèze valley by a siphon AND a trench of 11.5 km, perhaps due to malfunctions of the siphon. The techniques used — opus reticulatum, tile-proof mortar (opus signinum), and wooden formwork — demonstrate a Roman know-how adapted to local constraints. In Lyon, the aqueduct ended near Fort Saint-Irénée, after crossing the Col de Trion by a last siphon. Today, its remains, scattered between urban and rural areas, offer a unique historical and landscape route, studied and valued by associations such as Forez-Jarez.

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