Tides and the causeway
Mont-Saint-Michel tides and the causeway, explained
The coefficient system, the 2014 bridge, and what a grande marée actually is.
See dates and tours on Viator ↗A bay that empties and refills twice a day
The tidal range in the Bay of Mont-Saint-Michel is one of the largest in continental Europe, with the water level varying by roughly 14 metres between low and high water on the strongest tides. The sea pulls back kilometres across the flats and returns at a pace that outstrips a casual walk, which is the root of most of the bay's old warnings to travellers.
The coefficient scale
French tide tables rate every tide on a coefficient scale from 20 to 120 — the higher the number, the bigger the gap between low and high water. The strongest tides, the grandes marées, cluster around the March and September equinoxes.
Why the bridge changed everything
From 1879 the Mont was linked to the mainland by a raised causeway road, which stopped the tide scouring away the silt around the rock. The bridge that replaced it in 2014, on slender piles designed by Dietmar Feichtinger, lets water flow freely beneath it — which keeps the crossing open on ordinary tides and helps the bay keep the Mont an island.
When the Mont becomes an island again
Only when the coefficient climbs to roughly 110 or above does water rise far enough to cover the bridge approaches and cut the rock off for a short time around high water — an event announced in advance on the tide calendar rather than a daily risk. On 21 March 2015 an exceptional tide submerged the bridge entirely. For an ordinary visit on an ordinary day, the tide never stops you getting on or off the rock.
Walking the causeway versus the shuttle
From the mainland car parks the walk takes 40–50 minutes; the free shuttle, Le Passeur, takes about 12. Walking at least one direction is worth it, since the abbey's silhouette changes the whole way.
Checking before you travel
SHOM, France's hydrographic and oceanographic service, publishes tide coefficients well in advance, and the Mont-Saint-Michel tourism office republishes the high-tide dates in a more visitor-friendly format — worth a look before fixing a time of day, especially around the equinoxes.