What is a tidal bore?
A tidal bore is a wave that travels up a river against the current when the tide starts to rise. Instead of rising gradually, sea water rushes into the river all at once and forms a front — a single wave or a train of waves — that moves upstream.
It has many names: bore or aegir in English, mascaret in French, pororoca on the Amazon.
Why does it form?
According to Hubert Chanson, of the University of Queensland, who has studied tidal bores around the world, three things are needed:
- A very large tide: bores appear at spring tides, when the range between low and high water exceeds 4 to 6 metres.
- A funnel-shaped mouth: the tide enters through a wide mouth that narrows upstream.
- Shallow water: in shallow water the wave steepens and breaks, as on a beach.
On the Severn, the channel narrows and gets shallower upstream, which squeezes the tide upwards and forwards.
Where can I see one?
- River Severn (England). The most famous in Europe. The largest reach about 2 metres high and travel at about 16–21 km/h. The big ones come in spring and autumn, near the equinoxes, according to the Gloucestershire Archives.
- River Seine (France). Its mascaret once exceeded 2 metres, but training walls and dredging made it almost disappear. Its last spectacular appearance was in 1963, although small bores still form upstream.
- Bay of Mont-Saint-Michel (France). A small bore, a few tens of centimetres high, runs up the Sée, Sélune and Couesnon rivers.
- Amazon (Brazil). The pororoca, already described by the first European explorers.
When is the best time to see one?
Tidal bores need big tides, so look for the days of biggest spring tides — in France, the highest tide coefficients — and check the times published locally for each river.
What is a tidal bore?
A wave that travels up a river against the current when the tide starts to rise.
Where is the most famous tidal bore?
In Europe, on the River Severn, in England, where the largest are about 2 metres high.
When do tidal bores happen?
At spring tides, when the range of the tide is very large, especially near the equinoxes.
Sources: H. Chanson (University of Queensland), Tidal Bores, Aegir, Eagre, Mascaret, Pororoca (World Scientific, 2012) · Gloucestershire Archives — Severn Bore factsheet · Bonneton et al. (2012), Comptes Rendus Geoscience 344 · Mont Saint-Michel Normandie tourist office · H. Chanson — Tidal bores