An unseen hand reaches down from the sky to pull the water upstream, like a duvet being drawn up on a bed. In the course of 30 seconds the river's flow reverses to become a surging torrent moving in the wrong direction. The Severn Bore storms past as we stand watching.
This phenomenon is not unique to the Severn, but it is pretty rare. Half a dozen rivers around the world produce comparable surges, but this one is easy to get to and definitely worth the trip. Some people are disappointed when it fails to produce a full wave. But the event is dramatic anyway.
The Bore comes up the River Severn every month, but is usually a minor event. At the spring tides closest to the Spring and Autumn equinoxes it can be far more dramatic. Bores are rated from one to five-star and the five-star Bore can be two metres high. If it forms a wave across the width of the river it can be truly amazing, but a 6-7 foot surge is quite a phenomenon even when the front end is wedge shaped. Once the front has passed the tide continues its rapid rise providing the spectacle of a complete tidal sequence in less than an hour. The Bristol Channel, which funnels into the mouth of the Severn, enjoys one of the greatest tidal variations in the world. The water at Avonmouth can rise 48 feet between low and high tides. Little wonder that this volume of water produces odd effects upriver.
The Severn Bore is caused by a massive tidal surge. Look at the southwest corner of England and you will see the problem the water faces as the gravitational pull of the moon draws it into this dead-end street. Trapped between Wales and the Southwest peninsular, the tide is progressively compressed into a smaller and smaller area. The estuary is quite wide where the M48 bridge crosses the Severn and the river Wye branches off into Wales, so the water continues to flow upstream into this broad and inviting space. By the time it reaches Newnham-on-Severn the tide is committed and has nowhere else to go. From here on it is forced to climb onto its own back - much to the delight of the surfers.
Newnham is the point where Bore surfing begins. Watch from there an hour or so before the wave is due to pass Minsterworth and you will see dozens of wetsuited hopefuls hovering in the muddy river waiting to ride the oncoming Bore. The banks are still far apart at that point, so the Bore has yet to reach its peak; but it has to start somewhere.
The next popular viewing point is beside the Severn Bore pub a couple of miles south of Minsterworth. On a five-star day this riverbank may be crowded, especially if the phenomenon is obliging enough to come at a weekend and at a civilised time. Minsterworth is the first of the classic viewpoints and the earliest Bore arrival point that the published timetables mention. A footpath runs beside the parish church and out to the riverside where you can get good views of the Bore and enjoy a pleasant panorama of Gloucestershire scenery. If a high Bore is expected you should stand back from the bank to avoid being swept away. This may be a small river but, when thousands of tons of water rush past you at running speed, you should take it seriously. The actual speed is about 10 miles per hour (16 kilometres per hour) so you can outrun it in a car.
The river winds its way through a far more tortuous route than the road, so it is possible to view the event at Newnham, motor slowly up the road to catch it at Minsterworth an hour later and still drive moderately to Over Bridge (near Gloucester) where it arrives after another half an hour. Between Minsterworth and Over Bridge the Bore passes Stonebench, which is the third point mentioned in the timetables. However, Stonebench is on the eastern side of the river and there is no chance of combining that viewpoint with the other popular spots.
People travel miles to see this phenomenon and some are disappointed. Overblown reports lead them to anticipate a 6-foot wall of water sweeping up the river before their eyes. It is possible, but infrequent. Go with more moderate expectations and you will see plenty to remember. Preferably go twice - once to see the event pass two or three viewpoints and once to linger for an hour at one location and watch the waters rise higher and higher bearing their cargo of branches, driftwood, near-complete trees and assorted rubbish. It is said that some of this detritus has passed up and downstream for twenty years. Whoever made this observation deserves an award for patient endurance, but I'll accept their word for it.
For me, the Severn Bore is a natural wonder to marvel at and enjoy. If it happens at an unsocial hour it is still worth the effort. When I return home to catch up with my sleep I'll recall that unseen hand pulling the river upstream, just as I draw the warm duvet over me.