I’ve lived in Gloucestershire for nearly 25 years, but until this month I’d never witnessed the Severn Bore. This is the phenomenon of an inrushing wave that sweeps along the river as it narrows from Sharpness all the way to the first weirs just past Gloucester. The bore is caused by the incoming rising sea water being channelled into a swiftly narrowing river.

A bore can occur with any Spring high tide (that is, the higher-high-tides that occur around the full and dark moon). But the biggest bores occur around the equinoxes, when the sun’s and moon’s gravitational effects combined are at their greatest. The very biggest – five star – bores occur when all factors align, including meteorological components.

On Wednesday 22 February, at 10:03 I watched the (four star) bore arrive at Over Bridge, near Gloucester. You’ll be disappointed if you are expecting a tsunami. The wave is maybe 30-50cm in height. It seems oddly slow-moving.

But it is clearly an unstoppable force. Before it arrives the river is placid and flowing at a good pace down stream. The bore advances, and in an instant the flow is reversed, and water is piling up-stream. Canoeists who previously were having to paddle to maintain their position against the down-stream current, are in an instant picked up and swept upstream – effortlessly on their part.

On Thursday there was another four star bore, which this time I watched at 09:07 at Newnham. The river here is uncoiling like a massive python, looping and broadening as it reaches towards the sea. So the inrushing tide is still in a wide channel, but one that has nevertheless narrowed significantly as the loops have been negotiated. We watched surfers and kayakers from far off holding the wave and effortlessly pass us by, possibly travelling many more miles upstream.