In the 19th century, the Couesnon was channelled to put an end to its destructive “wanderings” that were eroding the shores. This action fostered the silting up of the bay and the formation of polders. In 1969, the Caserne dam came into service on the Couesnon to prevent flooding of agricultural land. During 40 years, the gates of the dam blocked the inflow of the rising tide. Hundreds of thousands of cubic metres of sediment accumulated downstream and upstream of the structure. Nearly 0,15 sq. miles (40 hectares) of maritime space were thus reclaimed from the sea within a 1-km circle around the rock.