How Is a Tidal Bore Produced?

A tidal bore is a wave which travels up a river. A large tidal bore is a spectacular sight, drawing both spectators and surfers. According to the UK's National Oceanography Centre, there are approximately 100 rivers worldwide that produce tidal bores.

Cause of a Tidal Bore

• Tidal bores are produced where there is a large tidal range at a river estuary. They occur during a flood tide when an immense volume of water is forced from the ocean into the narrower, shallower channel of an estuary. A wave, or bore, is produced and funnelled upriver. As the river decreases in width and depth, the wave becomes higher. The largest bores occur at the vernal and autumn equinoxes, one to three days after a full or new moon.

Features of a Tidal Bore

• A tidal bore can be a single, smooth wave or a succession of turbulent waves. The wave is often preceded by a low, rumbling sound, caused by air bubbles and the disturbance of silt and small stones. The Qiantang River in southeast China has the highest tidal bore, reaching 30 feet (9 meters).

Where Tidal Bores Occur

• Many tidal bores occur in Europe, the most famous at the River Severn, UK, with the highest wave recorded at 9.2 feet (2.8 meters). In North America, tidal bores take place at the Turnagain Arm of the Cook Inlet, Alaska, and at Fundy Bay, Gulf of Maine.