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Origin and history

The Gier's aqueduct, built in the second century under the reigns of Trajan and Hadrian, is one of four ancient aqueducts feeding Lugdunum (Lyon). With its 85 km, it is the longest and best preserved. It draws its waters from the river Gier (fluent of the Rhône) and crosses the Rhône department, passing through Mornant, Chaponost, and Sainte-Foy-lès-Lyon. Its route combines tunnels, bridges-siphons (like that of Beaunant, classified in 1875), and aerial sections in opus reticulatum, rare technique in France.

The dating of the pipe was debated: initially attributed to Claude (I century) after the discovery of lead pipes in his name, it was revised thanks to the "stone of Chagnon" (1887), a Hadrian stele (117–138 A.D.) prohibiting crops near the structure to protect it. In 2018, dendrochronological analyses on wooden formwork of the Beaunant Siphon Bridge confirmed a construction around 110 AD, under Trajan, with a possible completion under Hadrian. These discoveries ended a century of speculation.

Aqueduct uses a variety of Roman techniques: 73 km of covered trenches, 12 tunnels (one of which is 825 m near Mornant), 4 siphons to cross the valleys (Durèze, Garon, Yzeron), and 90 viewing glances spaced according to Vitruve's recommendations. In Mornant, an 825 m tunnel and a Mornantet bridge (arches vestiges) bear witness to its passage. The Yzeron siphon, with its 12 lead pipes under 13 pressure bars, represents a technical feat to cross a depression of 3 km.

Protected since the 19th century, the aqueduct has benefited from recent restorations, such as in Chaponost (2009–2010), where arches in opus reticulatum and reservoirs have been consolidated. In 2018, he was selected for the Heritage Lotto. Two Milestones (stones of Chagnon and the Rieu) recall the Roman regulations of protection, taken from a law of Augustus (11–9 BC).

Historical studies have evolved from the first surveys of Lyon antiques (XVIth-15th centuries), such as Guillaume Marie Delorme (1760) or Paul de Gasparin (1855), to the founding thesis of Germain de Montauzan (1908), which precisely mapped the route. In the 21st century, research focuses on missing looks (less than 100 identified on a thousand estimates) and construction techniques, such as the use of waterproof mortar (opus signinum).

Today, the Gier Watershed is an emblematic monument of the Gallo-Roman heritage in Auvergne-Rhône-Alpes, symbol of the Roman technical heritage and the sustainable management of water. Its remains, scattered over 18 communes, have been protected under the title of Historic Monuments since 1875 (Beaunant's bridge-siphon) until 1986 (Crest piles).

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