Rivers Three small rivers end in the bay (and cross it at low tide): the
Couesnon, now blocked to the west of
Mont Saint-Michel by a
causeway, the
Sée and the
Sélune. The very low slope of the bay and the very large
tides formed a
mascaret in those rivers that can travel upstream for many kilometers. Three larger rivers end in the bay: the
Sélune, the
Sée and mainly the
Couesnon that, accordingly to some local folk stories, was originally the boundary between
Normandy and
Brittany but then moved to the West of the Mont placing it in Normandy. In reality the boundary is not at the river location but further to the west, at the foot of Mont
Saint-Broladre.
Biggest tides in Europe Some of the biggest
tides in Europe occur in Mont-Saint-Michel Bay: with an average amplitude, they reach in average and up to during the highest
tides. This is due in part to the low depth of the bay and the barrier effect from the
Cotentin Peninsula. The ocean moves very fast, both at low and at high
tide, up to away. The
tides have been described by
Victor Hugo as "''à la vitesse d'un cheval au galop'' (as swiftly as a galloping horse)". In reality the full extent of the
tides is only visible at the entry of the
bay (the sea level around the
mont Saint-Michel is above the low
tide level) and its actual speed is closer to a walking human, the fastest observed being around 6.1 km/h. The
tides mix the water, creating a rich local
ecosystem. The
intertidal zone, the coastal area affected by the tides is about 200 km2 wide. During the highest tide, a
tidal bore can be observedless so since the bay has been under restoration work. == Levee and polders ==