Seen from afar, the Mont Saint Michel looks likes a gigantic sandcastle, permanently stranded at low tide. Closer, in summer-time, it resembles Disneyland. The Mont's single, narrow, medieval street is sometimes so jammed with tourists you have the impression that you could move along without your feet touching the ground.
In the 8th century, when a chapel was carved into its granite pinnacle, the Mont was a rocky island, three miles from the shore of the bay where Normandy and Brittany join. Now, on summer days, the car-parks and coach-parks sprawl like multi-coloured seaweed over the mud-flats on the landward side. The incoming sea scarcely ever wets the visitors' tyres.
Tide by tide, grain by grain of sand, that is beginning to change. A new dam has begun to operate at the mouth of the river Couesnon, two miles from the Mont Saint Michel. In one of the most ambitious and ingenious environmental projects ever attempted, the low, curving barrier has been designed to capture, store and then release, the combined power of river and tide. Once or twice a day, through a series of "flushing" actions, like an immense toilet cistern, the dam will gradually sweep away the silt and sand between the Mont and the "continent".
The project is still in its testing phase before its official opening in September but the mud-banks on the seaward side of the dam are already beginning to crumble. The river bed, as it meanders through the bay, has already widened in places from 20 metres to 200 metres. In five years, if all goes to plan, the road causeway to the Mont Saint Michel – the most visited site in provincial France – will be demolished and replaced by a bridge. Car-parks and coach-parks will be banished half a mile inland. By 2020, the Mont will become a true island once again, for the first time in more than a century.
President Nicolas Sarkozy was to visit the Mont Saint Michel this week to celebrate the completion of the first stage of the project. His brief collapse while jogging postponed the visit until September. The plan to "restore the maritime character" of the Mont Saint Michel is ambitious, almost pharaonic.
The flushing action of the dam will clear three million cubic metres of silt and sand, enough to stretch a metre deep, from Normandy to West Africa. If successful, the project could become a model for brain-led, rather than muscle-led, solutions to environmental problems all over the world. Francois-Xavier de Beaulaincourt, 53, the director of the agency which runs the project, says the works are immense, and at the same time gentle, unassuming, and not especially expensive at €200m (£170m). "This is actually a modest project, a humble project," he said. "By modest, I mean that the cost is not vast, equivalent to only 40km of new motorway. By humble, I mean this is not a question of man imposing his will and vision on nature. It is a question of man recognising his past mistakes and using nature, working with nature, to put things right, to put the clock back."
The island status of the Mont St Michel has been compromised by more than two centuries of human interference, from Dutch-style polders reclaimed from the bay in the 18th and 19th centuries to the tall dyke built to connect the abbey-rock to the "continent" in 1879. Getting rid of the mud and sand to restore the true majesty of the Mont is not especially controversial. Controlling the tourist tide which engulfs the rock each summer (more than three million visitors a year) is deeply contentious. The huge commercial interests sustained by the saintly rock are divided into two warring camps. One mostly controls the shops, restaurants and hotels on the island. The rival camp has tens of millions of euros invested in what the locals call "Las Vegas", the sprawl of shops, supermarkets, restaurants and hotels on the "continent" side of the dyke. Both sides – but especially the island interests – are fearful of the commercial implications of demolishing the dyke in 2014.
Cars and coaches will no longer be allowed to drive up to the island. They will be housed in new car-parks, hidden half a mile inland. Visitors will reach the Mont either on foot or aboard shuttle buses which will cross a low, unobtrusive bridge. The off-shore commercial camp, led by the Mayor of Mont Saint Michel, Eric Vannier, suspects that there is a hidden agenda to reduce the number of visitors to the island by making high charges for the car-parks and shuttle buses. "The Mont must remain a destination accessible to everyone," he said. "We are not doing all this work just for seagulls."
Mr de Beaulaincourt says the charges will be reasonable. There will still be plenty of tourists, and tourist euros, to please everyone after the ugly car-parks are swept from the dried-up sea-bed at the foot of the Mont. The intention, he says, is to "spread three million people a year over four square kilometres, rather than half a square kilometre".
Efforts will also be made to remind tourists that they are visiting not an off-shore, medieval shopping mall but a rock which has been a place of religious pilgrimage for more than 1,000 years. The 12th-century abbey constructed on top of the island, and carved far down into the granite, was one of the first entries in the Unesco list of the world's cultural heritage in 1979. As things stand, fewer than one in three of the visitors to the Mont bothers to climb the steep steps or ramparts to the abbey. Two-thirds squash into the shop-and-restaurant-lined village street, which is scarcely a metre wide.
The new, discreet dam, finished in May, could attract visitors of its own. It has an elegant viewing platform, with a wonderful panorama of the bay and the Mont, free of charge, from mid-September. Visitors hoping to see a surge of water, like an artificial Severn bore, when the dam is "flushed" will be disappointed. The combined river water and sea-water, trapped behind the dam at high tide, will be released gently over a space of two to five hours as the tide recedes. The effect will be to make the outgoing tide more powerful, and prolonged, than the incoming tide, gradually sweeping the sand and silt away.
The controlled release will be more effective than a surge. A slow release is also necessary to protect the many people who walk and wade across the bay to the Mont Saint Michel at low tide. Even with the gradual "flushing of the dam", pilgrims and other walkers will find that, in places, the water can rise rapidly from their knees to their hips.
Mathilde Charon, of the agency which is running the project said: "It was always advisable to cross the bay only with a proper guide. Now that is more true than ever. We work with the guides to explain what to expect and to let them know when the dam will be pushing out water."
Ms Charon believed the project would attract people to return to the Mont Saint Michel again and again. "A young person in their 20s visiting the Mont today will see a panorama which is very beautiful," she said. "But if they come back, perhaps with their own small children, 10 years from now, they will see something more natural, and even more spectacular."