Surfers ride the year's first 'four-star' Severn Bore

Hundreds of people gathered on the banks of the river Severn yesterday to witness one of Britain's most spectacular tidal phenomena.

The Severn Bore - a tidal wave that occurs in the lower reaches of the river upstream of the Port of Sharpness at the southern terminus of the Gloucester and Sharpness Canal - surged along the river in Gloucestershire and Worcestershire. The wave, together with one on Tuesday in Gloucestershire, wasthe first "four-star bore" of the year, though an unfavourable northern wind kept it relatively low.

The Severn Estuary receives the second highest tide anywhere in the world (only the Bay of Fundy in Canada is greater) and the difference between the lowest and highest tide in one day can be more than 14.5 metres. The high, or spring, tides occur on several days in each lunar cycle throughout spring and autumn.

Despite warnings from the local harbour authority, the tide (a self-reinforcing solitary wave, or soliton, as it is known to scientists) has long attracted surfers and yesterday was no different, with enthusiasts anticipating the kind of tide that enabled Steve King, a local railway engineer, to record a world record for the longest continuous surf last year. Although the tide did not quite meet expectations, it was, as usual, funnelled up the estuary into a wave that travelled rapidly upstream against the river current. The flood tide was accompanied by a rapid rise in the water level, which continued for about one and a half hours after the bore had passed.

"For a bore to form, a considerable rise in tide is needed in a converging channel with a rising riverbed, forming a funnel shape," an Environment Agency spokesman said.

"The size of the bore can be affected by wind direction, height of the incoming tide and fresh water moving downstream. It has been known to reach two metres in height and its average speed is 16 kilometres per hour."