They came from hundreds of miles away, rising long before dawn to stand on the riverbank in the springtime morning cold. About 40 people (plus a golden retriever) had gathered by the pub in woolly hats and anoraks. Similar groupings were at other vantage points downstream. They might have been a group of birdwatchers, alerted to a rare migrant, or simply commuters awaiting the morning train. Their quarry had something in common with Britain’s railways, being fickle and mysterious, frequently late and a letdown. It can also be early, which is a further complication.

We were waiting for what may well be Britain’s most remarkable natural phenomenon. In this country, nature tends to be temperate – if temperamental – and rarely offers the world’s biggest anything. And this isn’t quite the world’s biggest either. Not quite.

But the Severn Estuary does have the second-biggest tidal range in the world (behind the Bay of Fundy in Canada). That tide then funnels into the River Severn, which soon narrows and shelves. The upshot – on a handful of days a year – is the world’s second-biggest tidal bore (behind the Qiantang River in China).

The Severn Bore is well-enough known, and gives rise to plenty of bad puns. Yet there are surprisingly few initiates, even among those who live nearby. What happens is that Britain’s longest river, meandering its way seawards, suddenly meets – with a thrilling whoosh – the inrushing tide, producing a genuine tidal wave up to 2 metres high racing upstream at about 13mph. Then, for about an hour, the river flows the wrong way and keeps rising, sometimes flooding the nearby meadows and marooning imprudently parked cars, before it quietly subsides and normality is restored.

It is a spectacular phenomenon. It is also under threat. The government is considering plans to build a massive barrage across the Bristol Channel to generate vast amounts of electricity, which would scupper the Bore forever. Thus two fundamentals of the green movement’s agenda – creating renewable energy and respecting the forces of nature – find themselves in direct and seemingly irreconcilable conflict.

We were gathered at the Severn Bore Inn, just south of Gloucester. And here the 21st century had already intruded: the pub had closed down a month earlier, scuppering my hopes of the traditional pre-Bore breakfast fry-up.

But we were still excited. This was predicted to be one of the biggest Bores of the year. But the predictions (easily found on the web) are never exact and were confused slightly in 2009 because the Bore-watcher’s Bible, Arrowsmith’s Bristol Channel Tide Table, had changed the basis of its data for its 174th issue at the request of mariners, which subtly altered the timings.

There are about 180 visible waves a year, maybe 40 of them significant, coinciding with the spring tides of full and new moon. But only a handful will be really special and there are dozens of other factors that affect the size of the Bore, some long-term predictable (like the proximity of the moon to earth), some not (wind direction, the water level and – I reckon – whether the river’s just in a good mood or not).

The omens this morning were promising: word was borne upstream by surfers such as Rob Harris from Glamorgan, who had already had one ride down at Newnham-on-Severn and had raced up here for a second go: the wave moves fast but, barring roadworks, you can easily outrun it on the A48. This was a good one, he reported, exhilarated. Stand by.

The Bore virgins on the bank braced themselves. The Wilsons, Daphne and Allen, had got up at 4am to drive from Bideford in Devon, while Roger Huyshe had come down from north Shropshire. All three were fulfilling long-held ambitions. Julie Webb runs the Falmouth Surf School in Cornwall together with her husband, Spencer, who was in the water. A surfer for 40 years, his more energetic aim was to ride the Bore, not just see it. This was his moment. Julie and Spencer had been parked by the deserted pub the night before when the evening tide came in. Amidst the stillness, they could hear a distant roar. “Then it just rushed past,” she enthused.

“It’s coming!” said a voice suddenly. A speck of spume was visible past the bend in the river. And that was just about all we got – a speck. You could see the wave brushing the banks but over the rest of the mighty river …barely a ripple. “Is that it?” said Roger, with a look of pained disappointment. “Dead loss!” snorted Julie. I never saw the Wilsons again: missing, presumed heading back towards Devon in a huff. I’m lucky: I’ve seen it on a good day and it’s fantastic.

The expert verdict later was that the river level was too high, which had flattened out the wave. The verdict of some of the surfers was different. By luck or good judgment, a handful of them had headed for the far bank to catch the wave and got it right. “It was great!” said Rob. “Brilliant!” was Spencer’s verdict. “Only three of us caught it and unfortunately one got knocked off his board by a tree. But I got three or four minutes. It was quite some ride.”

We talked a bit about the barrage and what would happen if it were ever built. We agreed that nothing would happen for maybe a decade. He said it would be a shame to lose the Bore.

Some of those who know the river best think it would be more than a shame. “The Bore is crucial to the health of the river,” says Stuart Ballard of Save our Severn. “Not only does it flush it out twice a day, it keeps the silt in suspension. And the silt is an immense issue. It’s not a little bit of silt – it’s thousands of tons of it. Block that process and the river will clog up and just stop working. You’d get all kinds of problems, including more flooding upstream.”

The government has proposed five alternative schemes, two of them creating relatively small lagoons, three of them full-scale dams, the biggest of them 10 miles long and designed to produce one-seventh of Britain’s peak electricity consumption. It would also cost at least £22bn and block shipping into ports like Bristol.

And no one really knows what other unintended consequences, natural and economic, such a scheme might have. Ballard believes all the proposed schemes are obsolete and inappropriate for the Severn. He supports tidal stream turbines – far more efficient, he says. But even those would have some effect on the Bore.

In the meantime we’d better celebrate what we have. On any of those lists of “1,000 things to see before you die”, the Severn Bore would have to be included. Or alternatively the list of “1,000 things to see soon in case it dies before you do”. Or for Huyshe and the Wilsons, “1,000 things to see again on a better day”.

Ballard is organising a Magnificent Severn festival to coincide with the next expected superbores on the weekend of August 22-23. “I’ll definitely be there,” said Spencer Webb. “I’ll be aiming for a 10-minute ride.”

Matthew Engel’s dispatch appears fortnightly

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