Excavation of alluvial deposits of the Croult between its medieval stone-reinforced riverbanks. © UASD / O. Meyer.

Excavation of alluvial deposits of the Croult between its medieval stone-reinforced riverbanks.

© UASD / O. Meyer.

The Croult originated at Dugny, where a water supply point, known as trou provendier, regulated the flow of water. The canal then followed a seven-kilometer-long contour line before arriving at the abbey. In 869, its waters were used to fill the ditches of the castellum and, in 1356, those of the city-wall. Within the town, the canal split into two branches. The southern branch, or the Lower Croult, crossed through the monastery, the general hospital and the tanners' quarter, while the northern branch, or Upper Croult, ran parallel to the Rue Robert le Foulon and the "Poulies". The two branches came together again at the town's edge.

Several written sources tell us about the annual cleaning duties, and offer interesting technical details: the Croult was first drained by diverting its waters into the Rouillon, after which the mud was removed with ordinary tools such as shovels and large wicker baskets called mannes. Long, regular marks on the river bed may be ruts made by mud-hauling carts used during cleaning. After this man-made "drought", the river resumed its normal course.