Grass Planted to Stabilize Mud Has Profoundly Transformed Mont-Saint-Michel Bay

It resembles little more than a blade of grass beaten by the tides. Yet the English Spartina has an astonishing talent: to trap sediments and push the salt marshes forward. In Mont-Saint-Michel Bay, its history reveals, above all, how a landscape can tilt under the combined forces of nature and human interventions.

A weed from England appears in the bay and begins to encroach on the mudflats

At first glance, it is hard to imagine any threat. Spartina anglica grows in tufts on zones regularly covered by the sea. This robust grass is the product of a singular botanical history, born in England after hybridization between European Spartina and American Spartina. In France, it was introduced to Normandy at the beginning of the 20th century.

Its presence around Mont-Saint-Michel is recorded around 1925, according to a guide dedicated to the species. Only four years later did its expansion worry local officials. The plant indeed has a remarkable quality: it tolerates salt, immersion, and the loose soils where many other plants would struggle to take root.

But the story holds an important nuance. Natura 2000 documents devoted to the bay indicate that the populations of Spartina anglica do not necessarily occupy vast areas. A study published in Estuarine, Coastal and Shelf Science even described it as relatively rare in some western sectors. The scenario is therefore more complex than a generalized invasion.

Its stems slow the water and trigger a remarkable natural sediment trap

The plant’s true power lies almost at its feet. When the tide covers its tufts, leaves, and stems, it locally slows the currents. A portion of the suspended particles can then settle. The roots subsequently stabilize the substrate, gradually promoting the establishment of denser vegetation on certain pioneer areas.

This mechanism is well documented in Spartina species, although its intensity depends heavily on the hydrodynamic context. In other words, the plant does not manufacture the sediments itself. It acts more like a vegetal net, capable of catching a fraction of what the sea already transports. Through countless tides, a few centimeters can modify vegetation and the landscape.

This progression is not insignificant for animals. When open mudflats become vegetated, invertebrate communities can change, and with them the resources available to fish and birds. The bay remains, however, a major site for avifauna, hosting, according to the National Inventory of Natural Heritage, up to 200,000 to 250,000 waterfowl in winter.

The siltation of Mont hides a much larger story than a mere invasive plant

Blaming Spartina alone would, however, tell only half the story. UNESCO reminds us that siltation results from natural phenomena, but above all from developments that disrupted water circulation. The causeway-road built in 1879 ranks among the decisive transformations: it notably prevented water from circulating freely around the rock.

The channeling of the Couesnon, the poldering, and the old dam have also modified the estuary’s mechanics. When sediments accumulate sufficiently, the sandbanks can advance and lock in more material. Vegetation thus becomes one of the gears of a much larger sedimentary loop, and not the sole cause of a Mont gradually tied to the land.

To bring Mont back to the sea, the entire tidal machinery had to be rebuilt

Facing the risk that the rock could permanently lose its maritime character, an exceptional project was undertaken. Its baseline cost reached 184.74 million euros. A new dam on the Couesnon, the removal of the old causeway-dike, and the construction of a footbridge: the aim was to restore mobility to the water rather than simply remove tonnes of sand.

Put into operation in 2009, the dam can store up to 1.2 million cubic metres of water before releasing it back into the bay. These discharges re-suspend sediments and move them away from Mont. Since 2015, the rock can thus temporarily regain its island status during high tides, a spectacular result of decades of work.

The story of Spartina remains, in the end, fascinating for another reason. A plant a few tens of centimeters tall can contribute to reshaping a coastline when conditions are favorable. But the bay mainly reminds us that a landscape is a complex mechanism: vegetation, currents, sediments, and human works continually interact with each other. And sometimes, for more than a century.