The new dam at Mont Saint Michel controls the flow of water between the Couesnon estuary and the sea. It regulates the passing of the water, so that it drags the sand, preventing it from accumulating at the base of the island.
Since its construction, the dam has also contributed to the conservation of local flora and fauna.
Romain Desguée, who manages the dam, explained to Euronews how it works.
"Prior to the construction of the old dam, seawater naturally entered the estuary or returned to the sea. So, we had a lot of fish, seals, etc., which could enter the estuary. Something that the old dam didn't allow. With the new one, the sea water comes in and out of the river," he said.
"During the works we dredged the Couesnon estuary. This had an impact on the river because there were cane fields that had grown on its banks. They are the habitat of a certain species of birds, so we had to recreate this habitat in several parts of the bay, in order to compensate for this destruction.
"There are a lot of anecdotes. At one time, there was a seal that came before we let the sea water pass into the estuary. She would stand about 50 metres downstream and wait for the gates to open, to enter the estuary. She hunted in the estuary, and when we let the water out she would come out as well. She hunted several kilometers upriver and we saw her again a few days later, leaving towards the bay."