The Severn is subject to large and rapid rises in water level. On the lower reaches of the non-tidal part of the river, the rise occasioned by flood is often of the order of 20 feet, while the maximum tidal variation at Avonmouth can be 48 feet. This is greater than that of any other river in the British Isles and is exceeded only in one other place in the world the Bay of Fundy in Canada.
One of the outstanding features of the river is the phenomenon known as the ore, a word said to have been derived from the Scandinavian 'bara', a wave or billow. A combination of the considerable rise of tide, narrowing channel and other conditions in the estuary create the Bore.
A high bore may reverse the flow of the river as high up as Tewkesbury Lock, 13 miles above Gloucester. Bores occur at about the time of high water during the time of equinoctial tides.
Under most favourable weather conditions the Severn Bore can attain a height of five to six feet, and its average speed a few miles downstream of Gloucester is about ten miles per hour. The best viewing points are in the areas of Stonebench and Minsterworth.