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Origin and history

The Gier Waterway, 86 km long, is one of four ancient aqueducts serving Lugdunum (Lyon). Built probably during the reigns of Trajan or Hadrian (early 2nd century), it draws its waters from the Gier River, a tributary of the Rhône. Its remains, including siphon bridges and arches, have been protected since 1875. The precise dating was debated: discoveries such as the Stone of Chagnon (1887), bearing an edict from Hadrian, or wooden formwork dated 110 AD suggest a construction under Trajan, completed under Hadrian.

Aqueduct crosses 21 communes, using various techniques: covered trenches, tunnels (including one of 825 m near Mornant), canal bridges and siphons to cross the valleys. In Chagnon, the durèze siphon is lined with a 11.5 km canal, including an accessible tunnel called Cave du Curé. The route, studied from the 16th century by scholars like Guillaume Marie Delorme, was mapped precisely in the 19th century by Paul de Gasparin. Germain de Montauzan's thesis (1908) remains a reference for its completeness.

Recent excavations revealed technical details, such as the reticulatum (parly diamond) unique in France, or the use of lead pipes in siphons. Restorations, such as those of the Chaponost Arches (2009–2010), have preserved threatened sections. Aqueduct, classified as a historical monument by sections (1875 to 1986), illustrates Roman engineering and its adaptation to relief. Its estimated flow rate of 15,000 m3/day made it a vital infrastructure for ancient Lyon.

Two protective pillars, the Pierre de Chagnon (1887) and the Pierre du Rieu (1996), attest to strict regulations: prohibition of ploughing or building nearby to preserve the work. These inscriptions, similar to those of the aqueduct of Venafro (Italy), take up a Augustian law (11–9 B.C.). Despite gradual decommissioning, the aqueduct remains a major testimony of the Roman hydraulic heritage, studied and highlighted until today, as evidenced by its inclusion in the Heritage Lotto (2018).

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