“When the bore comes, the stream does not swell by degrees, as at other times, but rolls in with a head foaming and roaring, as though it were enraged by the opposition which it encounter” wrote Thomas Harrel in 1824.

One of the most exciting and interesting events to occur naturally on a river takes place throughout the year on the River Severn. Several times a month, a tidal wave makes its way along the river. It is known as the Severn Bore.

Tide waves (bores) naturally occur on rivers with large tidal ranges. This natural phenomenon occurs in the lower reaches of a few rivers throughout the world, during high tides. The name ‘bore’ as applied to the tidal phenomenon, appears to be derived from the Scandinavian or Icelandic ‘bara’ meaning a wave, swell or billow.

A bore is formed when the tide rises in a converging channel with a rising riverbed, forming a funnel shape. Large tides occur when the Moon and Sun are in line with the Earth at their closest respective distances above the Equator. The largest spring tides occur near the autumn and spring equinox, usually in March and October.

The Severn Estuary experiences the second highest tide anywhere in the world, and the difference between the lowest and highest tide in any one day can be more than 14.5 metres. These high or spring tides occur on several days in each lunar cycle throughout the year.

The size of the bore can be affected by opposing winds or high freshwater levels which reduce its height and delay its time of arrival, but a following wind can increase its height and speed up its arrival. It travels at an average speed of 16kph and has been known to reach an incredible 2 metres in height.

The bore can be seen along the lower lengths of the river to Gloucester. The incoming tide and bore can be seen, although not quite so impressively, in the upper estuary, at Epney, Newnham and Fretherne where times are 20, 60 and 85 minutes before Minsterworth respectively.

‘The big muddy’. One interesting feature which can be overlooked is after the bore wave has passed, the river level rises, presenting a slightly different aspect to the river scene, plus the river is flowing in the wrong direction. This continues for up to an hour after the wave has passed until the river magically appears to stand still before resuming its normal direction of flow.