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 five miles wide, then past Chepstow and Aust, then Lydney and Sharpness where it is approximately one 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.
The Severn Bore is one of Britain's few truly spectacular natural phenomenon. 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 eight in the UK) is one of the biggest in the world.