On the right tide, a British river can stop flowing and start travelling backwards: a single wave rolling upstream against the current, curling over the mud. A handful of times a year it is worth rearranging your week for; most months there is something modest to see if you know which stretch of bank to stand on. Here is where the UK's tidal bores appear, and how to work out when.

What turns a tide into a bore

A bore needs a funnel. As the incoming tide squeezes into an estuary that narrows and shallows, the leading edge of water is forced upwards because it has nowhere else to go. Instead of rising gently, it arrives as a step — a wall of water that pushes upstream against the river's outflow.

The Severn Estuary has one of the biggest tidal ranges on the planet, which is why its bore is the famous one. A river that has been running low for a week or two helps as well: the less fresh water pushing down, the less the wave has to fight. Heavy rain upstream can flatten a bore almost completely, which is why a promising tide sometimes produces nothing much at all.

The Severn Bore: Britain's headline act

The Severn Bore runs from the estuary up past Gloucester, and on the largest spring tides it can carry surfers and kayakers several miles in a single ride. On ordinary spring tides it is a low, polite ripple; on the biggest equinoctial tides it becomes a breaking wave worth the journey.

It arrives twice a day on the tides that produce it — roughly twelve and a half hours apart — though the two are rarely equal. Bore timetables published locally give predicted times for key points along the river.

Where to stand on the Severn

Newnham-on-Severn is a good first stop, with the wave forming out in the channel. Minsterworth is the classic viewing village, with a grassy bank and easy parking. Stonebench, near Elmore, gives a longer view of the bore's approach, and Over Bridge carries a crowd when a big one is forecast. Further upstream, the wave fades into a surge rather than a breaker.

The Trent Aegir

The Trent's bore takes its name from Aegir, the Old Norse sea god, and it behaves like a quieter cousin of the Severn's. The Trent is tidal as far as Cromwell Lock, well above Newark, and the wave is most often reported around Gainsborough, where the riverside gives a clear, safe view of the channel.

Other established spots include Keadby and Burringham, with smaller, fussier waves recorded further upstream at Collingham and Besthorpe. The Aegir is less predictable than the Severn Bore: it appears on big spring tides, tends to be more visible when the river is low, and can be generous one month and absent the next.

Smaller bores worth a detour

• Arnside Bore, Cumbria. A modest wave on the River Kent estuary, watched from Arnside promenade. It is a pleasant, low-effort outing — and the view across Morecambe Bay does a lot of the work.
• River Parrett, Somerset. A small bore on the biggest tides, usually spotted around Burrowbridge and downstream towards Bridgwater. Conditions have to line up.
• The Dee and the Wye. Both have reported small bores in their estuaries on exceptional tides. Treat them as a bonus rather than a plan.

How to time your visit

Bore-watching is a forecasting hobby as much as a spectator sport. Work through these in order:

1. Start with the moon. Bores need spring tides — the days around new and full moon, with the biggest of the year near the spring and autumn equinoxes, in March and September.
2. Check a tide table. Use the standard port nearest your chosen river, either a printed table or the Admiralty's online predictions.
3. Find a bore timetable. Local bore-watching groups publish predicted arrival times at specific points. These are estimates: expect the wave within a window of about twenty minutes either side.
4. Add roughly fifty minutes a day. Tides run later by about that much every day, so a bore that arrives at breakfast on Monday is a lunchtime event by the weekend.
5. Look at the river level. The Environment Agency publishes river level data. Low or steady water is good news; a spate after heavy rain usually means a washout.
6. Check the wind and the forecast. A steady onshore breeze helps push water into the estuary, and you will be standing still in it for a while.

Arrive thirty to forty-five minutes early. That sounds excessive until you have watched a bore arrive ahead of schedule while you were still crossing a field.

Watching safely

• Never walk out onto mudflats, sandbanks or exposed estuary bed to get a better angle. The tide comes in faster than it looks and the ground can be soft.
• Do not try to outrun a bore along a bank. The wave front travels at running speed and the water behind it keeps coming.
• Keep back from the water's edge, avoid crumbling banks and stay off weirs, sluices and lock structures.
• Keep dogs on a lead and children within arm's reach — riverbanks get slippery, especially in winter.
• Respect farmland and private moorings, and park so that residents and farm vehicles can still get past.

Planning a bore-watching trip

Pick your river first, then find a tide that suits a day out rather than the other way round. A big spring tide at Minsterworth can be paired with breakfast in Gloucester; the Trent Aegir fits neatly around a walk at Gainsborough; Arnside works well as a half-day with the bay in front of you.

Take binoculars, a flask and waterproof boots. Photographs of bores are best taken from slightly upstream of the wave, looking back down the channel, with something in the frame to give it scale — a moored boat, a bridge, a line of posts. And if the bore fails to show, as it sometimes does, remember that you have spent an hour beside a tidal river with a decent view. There are worse ways to spend a morning.

Photo: 마바사 / Pixabay