The Bay of Mont-Saint-Michel occupies a depression of about 500 square kilometres with the second-largest tidal range in Europe, from 10 to 11 metres on average, and up to 16 metres during the equinox tides. Three main rivers flow into the bay: the Couesnon, the Sée and the Sélune. The intertidal zone, with an area of more than 240 km², is composed of a mosaic of mudflats, hermella reefs and salt marshes; there are also peripheral marshes, dune systems, cliffs and two rocky islets, including the one on which the Abbey of Mont-Saint-Michel was built between the 11th and 16th centuries. The salt marshes are among the largest in Europe and serve as habitat for 22 protected plant species. 100,000 or more waterbirds winter at the bay, including over 1% of the populations of red knot (Calidris canutus) and dunlin (Calidris alpina). The aquatic warbler (Acrocephalus paludicola) uses the Site as a migratory stopover, with several dozen individuals monitored each year, while the Balearic shearwater (Puffinus mauretanicus) is present during its moulting period. Marine mammals such as bottlenose dolphins (Tursiops truncatus), harbour porpoises (Phocoena phocoena), grey seals (Halichoerus grypus) and breeding harbour seals (Phoca vitulina) visit the Site; European eel (Anguilla anguilla) and brown skate (Raja undulata) are also found. A plan to restore the ecological continuity of the Sélune river with the levelling of two hydroelectric dams is underway.