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Origin and history

The Gier Waterway, 85 km long, is the most imposing of the four Roman aqueducts serving Lugdunum (Ancient Lyon). Powered by the Gier springs, tributary of the Rhône, it is distinguished by its remarkable state of conservation and its complex layout, combining trenches, tunnels, bridge-siphons and aerial arches. Its construction, originally attributed to Claude or Auguste, was specified in 2018 thanks to the dendrochronology: the foundations of Beaunant's bridge-siphon date from 110 AD, placing its realization under Trajan, with a possible finalization under Hadrian. The work demonstrates exceptional technical mastery, with an average slope of 1.1 m/km and an estimated flow rate of 15,000 m3/day.

The protection of the aqueduct was regulated by engraved pillars, such as the Pierre de Chagnon (1887) and the Pierre du Rieu (1996), prohibiting ploughing, planting or planting nearby. These inscriptions, dated from the Hadrian era, take up a Augustian law of 11-9 B.C. The route, studied in the 16th century by scholars like Pierre Sala or Guillaume du Choul, was mapped precisely in the 19th century by Paul de Gasparin and Germain de Montauzan. The latter, in his 1908 thesis, provided the most exhaustive description, highlighting its technical perfection and extension over 86 km, including 4 major siphons and 73 km of covered trenches.

Among the most spectacular remains are the Beaunant Siphon Bridge (classified in 1875), crossing the Yzeron with a elevation of 140 m, and the arches of the Air Plat in Chaponost, adorned with a rare cross-linked wall. The Soucieu Reservoir (classified in 1930) and the Cresta batteries (1986) complete the protected sections. Recent restorations, such as those of the Arches of Chaponost (2009-2010), have preserved these structures using Roman techniques. Aqueduct, selected for the Heritage Lotto in 2018, remains a symbol of ancient engineering and a conservation issue.

Its route, from the heights of Saint-Chamond (Loire) to Fourvière, combines the relief with various technical solutions: tunnels of 825 m (Mornant), bridges-channels (Jurieux, Granges), and visits spaced according to Vitruve's recommendations. A peculiarity intrigues archaeologists: the redundant bypass of the Durèze valley by a siphon and a trench of 11.5 km, perhaps due to initial malfunctions. Materials, such as the reticulatum or the signinum, reveal a rare Italian influence in Gaul.

The history of its decommissioning and rediscovery reflects the evolution of knowledge. From the 16th century, antique dealers such as Symphorien Champier or Claude de Bellièvre mentioned its existence, but it was in the 18th century, faced with water shortages, that the Académie de Lyon revived its study. The works of Delorme (1760) and Flacheron (1840) lay the foundations of modern research, while excavations of the 21st century (2018 in Saint Joseph) still reveal buried sections. Today, less than 100 looks at the estimated 1,000 have been located, leaving a part of mystery to this emblematic monument.

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