The Severn Bore is one of Britain's few truly spectacular natural phenomena. It is a large surge wave that can be seen in the estuary of the River Severn, where the tidal range is the 2nd highest in the world, being as much as 50 feet (approx. 15.4m).
As many as 60 bores occur throughout the world where the river estuary is the right shape and the tidal conditions are such that the wave is able to form. The Severn Bore (one of 8 in the UK) is one of the biggest in the world but bores also occur on the Seine and Gironde in France, on the Indus, Hooghly and Brahmaputra in India, on the Amazon in Brazil, on the Petitcodiac, New Brunswick, and also the Knik Arm bore at the head of Cook Inlet, Alaska. By far the biggest bore in the World is the Ch'ient'ang'kian (Hang-chou-fe) in China. At spring tides the wave attains a height of up to 25 ft (7.5 m) and a speed of 13-15 knots (24-27 km/h). It is heard advancing at a range of 14 miles (22 km).
The shape of the Severn estuary is such that the water is funnelled into an increasingly narrow channel as the tide rises, thus forming the large wave. The river's course takes it past Avonmouth where it is approximately 5 miles wide, then past Chepstow and Aust, then Lydney and Sharpness where it is approximately 1 mile wide, and soon the river is down to a width of a few hundred yards. By the time the river reaches Minsterworth it is less than a hundred yards across, maintaining this width all the way to Gloucester.
As well as the width of the river decreasing rapidly, then so does the depth of the river also change rapidly, thereby forming a funnel shape. Therefore as the incoming tide travels up the estuary, it is routed into an ever decreasing channel. Consequently the surge wave or bore is formed.
The best months for viewing this are January - March, and August - October. For more information look at the environment agency website