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Origin and history

The "Rhône" (Rhône, Auvergne-Rhône-Alpes) is a vestige of the imposing Gier aqueduct built in the second century under the Roman Empire to feed Lugdunum (Lyon). This 85 km long structure – the longest of the four Lyon aqueducts – drew its water from the Gier River (fluent of the Rhone) and combined covered trenches, tunnels, bridge-siphons and aerial sections. Its characteristic square piles combine a blocking heart and a trimming of crosslinked bellows interrupted by brick seats, rare technique in France outside of central Italy.

The precise dating of the aqueduct caused debate: initially attributed to Claude (I century) via lead pipes discovered at Fourvière, it was revised after the finding in 1887 of the Stone of Chagnon, an edict of Hadrian (II century) prohibiting tillage and plantation near the structure to protect it. In 2018, wooden formwork dated by dendrochronology of Year 110 (the Trajan Reign) in the foundations of the Beaunant Siphon Bridge confirmed a construction started under Trajan, possibly completed under Hadrian. The aqueduct, which was disused much later, was studied in the 16th century by Lyon scholars such as Pierre Sala or Guillaume du Choul.

The section of Sainte-Foy-lès-Lyon, classified as a Historical Monument (partial protections as early as 1875, 1912, 1930 and 1986), includes the spectacular Beaunant Siphon Bridge (siphon of the Yzeron), a 270 m long structure and 17 m high using 12 lead pipes to cross a 140 m depression. This siphon, with its 13 pressure bars, demonstrates exceptional hydraulic know-how. The remains still visible (reservoirs, crawlers, arches of the Air Course in Chaponost) are protected, although some sections, such as the one moved to Chaponost in 2006 to avoid its destruction, suffer from erosion.

In the 19th century, the Gier's canal, which was mapped by Paul de Gasparin, used almost all Roman techniques: 73 km of covered trenches, 12 tunnels (one of which was 825 m near Mornant), 40 canal bridges, and 4 major siphons. A peculiarity intrigues archaeologists: the redundant bypass of the Durèze valley by an additional 10 km trench, suggesting malfunctions of the original siphon. The visit looks, on average 77 m apart (in line with Vitruve's recommendations), were of the number of a thousand originally.

The ancient regulation surrounding the aqueduct is known via two engraved pillars, the Pierre de Chagnon (1887) and the Pierre du Rieu (1996, Saint Joseph), bearing an imperial edict prohibiting any agricultural activity nearby to preserve the work. This text contains an Augustian law of 11-9 B.C., confirming the strategic importance of the water system. Recent restorations, such as those of Chaponost arches in 2009-2010 (led by Didier Repellin), have enabled the restoration of original Roman techniques, using waterproof tile-based mortars (opus signinum).

Today, the Gier aqueduct enjoys multiple protections under the Historical Monuments, with sections classified in the communes of Sainte-Foy-lès-Lyon, Soucieu-en-Jarrest, Chaponost, Mornant, Brignais and Lyon. Its route, which is partially accessible to hikers (aqueduct), remains a major testimony of Roman civil engineering in Gaul, while still asking questions to archaeologists, such as the exact chronology of its construction or the reasons for its technical redundancy.

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