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Origin and history

Arcier's aqueduct, sometimes called Besançon's aqueduct, was an 11 km Gallo-Roman structure built between the hamlet of Arcier (Vaire commune) and the Castan Square in Besançon. Mostly buried or semi-entered, he followed the level curves along the Doubs to limit the development. Its 0.22% slope allowed water to flow from the sources of Arcier, the only ones able to feed the city, to a castelum divisorium (distribution tank) in the current Castan Square. The canal, 0.75 to 0.85 m wide and 1.55 to 1.62 m high, entered Besançon by an opening cut into the rock, future Tailed Gate.

The construction would date either from the Vespasian era (ca. 70 AD) or from Marc Aurèle (ca. 170 AD), with a privileged hypothesis under the Flavians (second half of the first century). This ambitious, costly and technically advanced project would have received imperial support in recognition of the loyalty of the Sequanes (Gaulian people of Vesontio, present Besançon) during the troubles of 68–70 AD. The aqueduct functioned until the fifth century, before being cut during the barbaric invasions and then abandoned. Its remains, partially visible along the D323 (between Chalèze and Vaire) or under the Castan square, testify to construction techniques adapted to the terrain: trenches, sizes in the rock, and substructures laid to cross the valleys.

Among the sections preserved, that of the Combe à Chalèze reveals a wall wide of 3 m, pierced by an arch and supporting the canal, with visible interior arrangements ( vault, hydraulic mortar, maintenance hatches). The partially preserved castella divisorium distributed water in the city. Two restoration projects were envisaged in the 17th and 19th centuries, but only the second one resulted: between 1850 and 1855, Besançon built a new aqueduct to capture the waters of Arcier, as evidenced by commemorative plaques near the spring and on the Saint John reservoir, designed by architect Alphonse Delacroix. Today, these waters still feed part of the city, treated since 1935 by the Malate factory.

Ranked as historical monuments by decree of 22 October 2021, the aqueduct crosses four communes (Vaire, Chalèze, Montfaucon, Besançon). Its remains, scattered on private or public plots, illustrate the hydraulic development of an ancient city capital. Unique aqueduct known for Vesontio, it compares with that of Montjeu (autun feeding), by its dimensions and techniques. Its layout, avoiding straight lines, combines level curves to minimize the work, reflecting a pragmatic engineering characteristic of the Romans.

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