The Aqueduct of the Gier is one of the ancient aqueducts of Lyon that supplied the Roman city of Lugdunum with water. It is the longest of the four aqueducts serving the city and is considered the best preserved of the group. The aqueduct takes its name from the sources of the Gier, a tributary of the Rhône, from which it drew water.
Its remains are protected as historical monuments under several classifications. The Beaunant siphon bridge was classified in 1875, the Soucieu reservoir in 1930, and the Crêtes piles in 1986. The aqueduct has also been the subject of long historical study. It was described by local antiquarians and scholars from the 16th century, and later studied and mapped by Guillaume Marie Delorme in the 18th century. Modern research has contributed to its dating and historical understanding, while its surviving structures continue to mark it as a major ancient engineering work in the urban heritage of Lyon.
The Aqueduct of the Gier
The Aqueduct of the Gier is one of the ancient aqueducts of Lyon that supplied the city of Lugdunum with water. It is the longest of the four aqueducts associated with this water supply system, and its surviving structures are the best preserved among them. The aqueduct is named for the sources of the Gier, a tributary of the Rhone, from which it drew water. Its remains are protected as historical monuments under several classifications. In 1875, the Beaunant siphon bridge was classified as a historical monument, followed by the Soucieu reservoir in 1930 and the Cretes piles in 1986.
Historical Study and Dating
The Aqueduct of the Gier was long the subject of hypotheses about its history and chronology. It was first attributed to Emperor Claude, based on lead pipes marked with his name found in Fourviere. In 1887, the discovery of the Chagnon stone led to the idea that construction may have taken place under Emperor Hadrien. This dating was later revived toward the era of Auguste after the fountain of Verbe Incarne was found with the inscription CLAVD AVG. More recent evidence came from the foundation of the Beaunant siphon bridge, where wooden formwork remains were dated by dendrochronology to the year 110 in 2018. The construction of the aqueduct was then placed under the reign of Trajan, possibly completed under Hadrien.
The aqueduct also has a long history of scholarly attention. Roman aqueducts in the area were mentioned by local antiquarians and scholars from the 16th century. Pierre Sala described them in the manuscript Les Antiquites de Lyon around 1520, and Symphorien Champier mentioned them in Petit traicte de la noblesse & anciennete de la ville de Lyon in 1529. From 1760, Guillaume Marie Delorme studied the aqueduct, published research, and mapped it at 1 scale. He established its profile through precise leveling and calculated the daily flow. His work was later referenced by Hippolyte Leymarie in 1838 and Alexandre Flacheron in 1840, and was recognized and published by the Lyon Academy of Sciences in 1855 under the title Reconnaissance de l'aqueduc romain. Accurate plan drawings by Gasparin and Flacheron also document its technical and architectural aspects. Germain de Montauzan reused elements of the aqueduct in his 1908 thesis, which laid the foundations for contemporary research and validated or invalidated earlier assertions. The aqueduct is described as the most extensive, prominent, and perfect of the four aqueducts feeding the ancient city, and its route includes a notable loop on the Chagnon commune.
Dating and Restoration History
The construction of the Aqueduct of the Gier is considered difficult to date because the epigraphic evidence is weak. A key element in its history was the discovery in 1887 of the Pierre de Chagnon inscription in the commune of Chagnon in Loire. This inscription is described as similar to one found in Italy and as carrying a general warning about a non-buildable zone around the structure. Archaeologist Armand Desbat dates the aqueduct to the Augustan era, and the available evidence indicates that the aqueduct is older than the time of Hadrian. According to the historical record, Hadrian only reaffirmed a law from 9 BCE concerning the protected zone, and the warning inscription itself was too imprecise for practical use. Under Hadrian, only a simple reminder of the law was needed after inspection and reorganization of the zone.
Restoration work on the arches in the commune of Chaponost was carried out in 2009 and 2010 with sponsorship support. The work was led by chief architect Didier Repellin and heritage architect Laurent Volay. It included restoring the original ground level, removing vegetation, eliminating old repairs, and carrying out masonry repairs using materials researched from Roman techniques. In May 2018, the Gier aqueduct was selected for the Heritage Lottery.
Protective Regulations and Inscriptions
Règlements governed the use of the aqueduct at Lugdunum, and this protection is represented by two markers known as the Pierre de Chagnon and the Pierre du Rieu. The Pierre de Chagnon is a sandstone plaque of unspecified size discovered in the Gier aqueduct sector near the Durereze valley bypass, while the Pierre du Rieu was discovered in 1996 in the municipality of Saint-Joseph. Both stones carry the same Latin inscription concerning aqueduct protection. The inscription prohibits plowing, sowing, or planting in areas reserved for aqueduct protection by order of Emperor Caesar Trajan Hadrian Augustus. It repeats terms of general legislation defined under Augustus in 11 and 9 BC, as recorded by Frontinus in his treatise on aqueducts. A similar inscription is known from the Venafro aqueduct in Italy, where it specifies construction setbacks.
Route and Surviving Structures
The route and description of the Aqueduct of the Gier make it one of the longest known Roman aqueducts, although its exact length is not specified. Its course has been well reconstructed from visible surface remains and the locations of many inspection shafts. It begins in the heights of Saint-Chamond in the Pilat massif, in the Loire department, then follows the relief of the plateau across the Rhone department, passing near Mornant, Orlienas, Chaponost, and Sainte-Foy-les-Lyon before ending in Lyon. Over its course, it descends 105 meters, with an average slope of 1.1 m per km, and is estimated to have carried 15,000 cubic meters of water per day. The aqueduct includes four siphon bridges and, unlike other aqueducts supplying the ancient city, it drew water directly from a river rather than a spring.
The entrance has been identified by Delorme as an underground tunnel on the left bank of the Gier river at La Martiniere in the Izieux suburb of Saint-Chamond, while Flacheron stated that the water was raised by a dam in the Gier riverbed. These entrance works no longer exist, and the dam at Moulin-Combat is considered the first structure of the aqueduct. The line then crosses the Janon valley at Pont Nantin on a 200-meter-long aqueduct bridge with about thirty arches, though its five still visible bridge pile bases were buried in 1998 during road widening. The main remaining structure in the area is the bridge over the Langonand valley, which had seven to nine piers, five of them still visible. Between Saint-Chamond and Genilac, the aqueduct follows the terrain and crossed several streams on bridges that are now destroyed.
A higher ditch, located about 14 to 15 meters above the aqueduct between Saint-Chamond and Saint-Maurice-sur-Dargoire, was first studied by Paul de Gasparin and is generally believed to be an initial trace abandoned because construction at that altitude was too difficult. Remains are known in three sectors. At the Collenon hamlet, the ditch is cut directly into rock and served as a conduit. At Leymieux, the aqueduct crossed the Dureze by a siphon bridge with a still-visible settling tank showing seven of the original ten lead pipe holes, and remaining piles are still visible in the Dureze valley. These ruins have been classified as a historical monument since 1962. The bridge over the Feloin valley had seven arches, but only the abutments remain. The siphon de la Dureze is also described as being doubled by a canal, with Germain de Montauzan cited for its construction after the siphon.
Visible Vestiges and Protected Remains
The Aqueduct of the Gier survives in several visible vestiges along its route. One passage is the tunnel known as Cave du Curé, located within a siphon, and the conduit inside it is also identified as the Cave du Curé conduit. Other remains can be seen on both sides of the Lachal path to Chagnon, with a third vestige reported in the wood near the Durze river. Hikers can follow the aqueduct trail to observe these remains.
Among the identified vestiges are a block from the Beaulieu bridge pile and the Fontanes tunnel. At Fontanes, three planks of Roman vault formwork are still preserved, and the tunnel is recorded as having a defined length, although the provided facts do not specify the figure. The aqueduct also reaches le Rieu commune after Saint-Martin-la-Plaine. Near the boundary of Saint-Martin-la-Plaine, a protection marker found in 1996 forbids plowing, sowing, or planting within six to fifteen feet around the aqueduct. A copy of this marker is displayed near the town hall.
Further downstream, the underground canal joins Saint-Didier-sous-Riverie near the pont du Bozanzon. It was punctured during agricultural works at Cartet in 2012, but the underground canal was found to be in good condition. The tunnel at Cartet is about twenty meters long between two collapses. In 2018, earthworks revealed two aqueduct bridge piles and thirty square-faced stones; one pile weighs nearly twenty tons and measures two meters on each side and two meters sixty high, and it was moved for conservation and presentation purposes. Observations of differences in field plant growth at Chourier have also been used to locate vestiges.
Surviving Structures Along the Route
The route of the Aqueduct of the Gier is marked by a series of surviving works that show both open-air and underground sections. Near Saint-Joseph, it crosses the deep valleys of the Grand and Petit Bozanco n by bridges or underground pipelines, while other sections remain underground on flat ground, except for an inspection well at Chemin du Loup. At Mornant, the aqueduct passes through a tunnel 825 meters long, and a 19-meter-deep inspection well is visible at La Bruyere. Further on, the canal can be seen on both sides of departmental road D 30, with an inspection well between them.
Several bridge and wall remains illustrate the former engineering of the canal. The most spectacular surviving element is the Jurieux canal bridge, described as the only complete bridge of the aqueduct. Other remains include the Granges canal bridge along the Bozanco n, the canal cut by the Chemin du Vieux Moulin, and the Merdanson canal bridge, of which only the last arch and a few scattered blocks remain. At the entrance to Soucieu-en-Jarrest, the Granchamp wall is about 140 meters long and carries the canal with a reticulated facing.
In Soucieu-en-Jarrest, the canal goes underground and then reappears on a wall and a series of arches, the last of which is called Le Chameau because of its shape. Another series of arches leads to the siphon of the Garon chase reservoir at La Gerle, where the canal and a tile-coated lining survive above an arch. The Pont-siphon du Garon ended with a leak reservoir at La Gagere, and the siphon bridge was classified as a historical monument in 1930. It originally had 23 arches, a length of about 200 meters, and a maximum height of 21 meters, but only 6 arches remain on the right bank and 4 on the left bank of the Garon.
Route, Structures, and Preserved Remains
The route and surviving structures of the Aqueduct of the Gier show a series of different engineering solutions along its course. At la Gagere, the leak reservoir retains the slope and support, and nearby there is a reticulated facing wall. This facing type is described as unique in France and rare outside central and southern Italy. The aqueduct crosses the Chaponost plateau supported by a wall or arches, and vestiges of this structure are visible throughout Chaponost village. One buried section includes an inspection opening.
At the Plat de l'Air site, the canal leaves Chaponost carried by 72 of the original 92 arches. The canal also crosses its last valley by the Yzeron siphon. The construction at Plat de l'Air combines stone, brick, and rubble. On the exterior, the parts have diamond-shaped rubble facing alternating with double layers of wide red bricks and inner brick edges for the arcades. The interior render shows pink waterproof mortar made with tuileau. The conduit and siphon reservoir at Plat de l'Air have been classified as historic monuments since 1900.
The Yzeron crossing at Beaunant, presented around 1901 to 1902, is described as the most spectacular and visible part of the aqueduct. Seen from downstream, several arches are missing because of D342 construction. The siphon was the only technical solution for crossing the Yzeron, where the aqueduct passes over a depression nearly wide with a drop of approximately . The remaining large vestiges consist of the barrage reservoir and the beginning of the inclined descent ramp, which is wide. These remains also allowed the lead pipe characteristics to be established.
A later episode in the aqueduct's preservation involved a buried section identified with an inspection opening. In response to OPAC du Rhone construction plans, a citizen group supported by sponsors and the commune decided to move the section. After a 2009 request for analysis to the Service departemental de l'architecture et du patrimoine, the department proposed several protection solutions, but the municipality chose not to protect the section and to wait for damage before reburying it. The moved section was surrounded by wood pieces and steel beams, lifted by a mobile crane, transported around a city block on a trailer, and placed at the new excavated site. It is now displayed near a parking area as communal heritage, although the exposed section deteriorates because it was designed for burial and has been left exposed to weather.
The Trion Siphon and the Final Crossing
The Aqueduct of the Gier crossed the Sainte-Foy-les-Lyon plateau and then had to pass a final depression at Col de Trion, south of Fourviere, through the Trion siphon. This siphon began with its barrage reservoir in Fort Saint-Irenee, where the support pile for the barrage reservoir and the descending ramp are located. The aqueduct crossed Col de Trion without using a siphon bridge, and the Trion siphon ended at a now missing leak reservoir on the current Rue Roger-Radisson. Foundations were found south of Sainte-Foy-les-Lyon for the siphon's leak reservoir, and remains of the Gier aqueduct are also recorded on Rue Roger-Radisson in Lyon. The route and surviving traces help locate the aqueduct's final section within the landscape of Lyon and its surroundings. The aqueduct piles are also shown within Fort Saint-Irenee, and Roman aqueducts on the Saint-Just heights appear in a 1806 painting by Jean-Michel Grobon now displayed in the Musee des Beaux-Arts de Lyon.
Construction Techniques and Valley Crossing
The Aqueduct of the Gier employs almost all Roman aqueduct construction techniques. One notable feature is its crossing of the Dureze valley, which combines a siphon with a trench that bypasses the valley. This detour increases the total length of the aqueduct. The use of both a siphon and a circumvention route has been considered a possible response to defective siphon operation, suggesting a redundant design approach.
Heritage Protection
Several sections of the Aqueduct of the Gier have been protected under the status of historical monuments since 1862. This designation reflects the recognized heritage value of the structure and its importance within the built landscape. The listing or classification of different sections indicates that the aqueduct has been formally preserved as part of the historical record over a long period of time.