Ancient canals in the valley of Bourgoin-La Verpillière (France, Isère): morphological and geoarchaeological studies of irrigation systems from the Iron Age to the Early Middle Ages (8th century bc –6th century ad )

Crop marks of long canals diverted from ancient rivers were identified on aerial photographs in the valley of Bourgoin-la Verpillière, in the northern part of the Isère department. The environmental multidisciplinary study of the sedimentary fill of these canals sheds light on their function and chronology of use between the Iron Age and the Early Middle Ages (8th century bc –6th century ad ). We interpret these hydraulic structures between 0.4 and 2 m deep, as part of large gravity irrigation systems for watering meadows, and even for growing specific crops in the marshes during the Roman period. These findings shed new light on the antiquity of irrigation practices in temperate climate zones and demonstrate the artificialization of wetlands since the Protohistory, based on a sophisticated engineering technology. These new data show that hydraulic systems were integrated into the Iron Age agricultural system several centuries before the Roman Conquest of the Transalpine Gaul in the end of the 2nd century bc .