Weirs: why you never swim near one

Weirs are part of the furniture on British rivers, and the slow, deep water above one can look like the perfect place for a swim. Below the weir is a different story. A weir on its own is a reason not to swim, and the Environment Agency, which manages weirs across England, says much the same.

What a weir is

The Environment Agency describes a weir as a low dam built across a river. Its own weirs help manage flood risk, water resources and navigation. Above a weir there are often slower, deeper pools; at the weir, the water spills over the top and falls. The Outdoor Swimming Society (OSS) says the same advice applies to millraces, where fast water flows out of a mill or an old mill site.

Weirs come in different shapes. The OSS mentions wide, curved weirs, and warns that U-shaped and box weirs are fatal to trapped swimmers and kayakers, because they drag everything into their centre. Its wider point is that weirs vary, so you can't judge one by another.

What happens below the weir

Water falling over a weir drives down to the riverbed, bounces back up and is drawn back into the water pouring over the top. The OSS calls this a stopper, a vertically recirculating current, and describes stoppers as extremely dangerous because they are hard to escape. The Environment Agency puts it plainly: stoppers can pull people underwater and trap them against the structure. The Canal & River Trust warns that weirs have currents strong enough to pull even the strongest swimmers under.

You'll also hear the word towback. A weir assessment system from Natural Resources Wales and Rescue 3 Europe, reproduced in a 2022 report by the Marine Accident Investigation Branch (MAIB), measures towback as the distance from the base of the stopper to the boil line, the point where the recirculating water comes back to the surface. In the accident that report investigated, the weir's closed-in sides, with no steps or a ladder, made it impossible for the people trapped in it to get out of the water.

Why you can't judge it from the bank

The danger sits under the surface. The OSS warns that stoppers can form in places that look safe, and that even a small weir can have a powerful one in summer. Seeing locals playing at the foot of a weir is no guarantee: the OSS says never assume it's safe, and judge it on that day.

The weir also changes with the river. The OSS says higher flow makes a stopper more likely, and that water gets faster and more forceful the closer you get to a weir. The MAIB report shows how far that can go. Two tour leaders checked their route in August, when conditions were benign, the weir was largely under water and the glassy, still surface showed almost no current. On the October morning of the tour they didn't go to look at the weir. A high river, a fast current and low tide had made it treacherous, and four paddleboarders on a commercial tour were "trapped at the weir by the hydraulic towback". All four died. The investigators found that the chance of that weir causing harm could swing from very unlikely to almost certain in a single day.

How far away is far enough?

None of the sources we checked gives a safe distance, so we won't invent one. The Environment Agency's advice is to stay well clear of weirs and always follow the safety signs. It also says not to walk across weirs, because fast-flowing water can knock you over, and it has said jumping or diving from its structures is prohibited.

Swimmers do use the slow pools above some weirs. The OSS points out that water picks up speed and force as it nears a weir, and that conditions change when the river is higher. After heavy rain, it advises avoiding weirs, waterfalls and mill races altogether. If it's rained recently, read river swimming after rain first.

The question to ask

Of all the questions to ask before a river swim, this is the one we'd least want you to argue with.

If someone ends up in the water

If you are caught in one, the OSS suggests trying to swim down or sideways to get out of the circulating water, then coming up to the surface, while being clear that some stoppers are impossible to escape. So all the advice comes down to not being there in the first place.

For what to say when you call, see who to call if it goes wrong. For the rest of a river swim, from reading the current to finding your way out, see the river swimming guide, and for other structures on the water, locks, gates and bridges.

Sources

1. Outdoor Swimming Society, Understanding Weirs
2. Outdoor Swimming Society, Understanding Wind and Rain
3. Environment Agency, Helping you choose where and when to swim in open water
4. Environment Agency, Weir removal, lowering and modification: a review of best practice
5. Marine Accident Investigation Branch, Commercial stand up paddleboarding accident on river weir with loss of 4 lives
6. Marine Accident Investigation Branch, Report on the investigation into the stand up paddleboarding accident that resulted in four fatalities at Haverfordwest Town Weir, Wales on 30 October 2021
7. Canal & River Trust, Summer holiday sunshine prompts us to ask you to stay out of the water
8. Environment Agency, Be aware of safety warning signs when swimming near water structures this summer
9. Environment Agency, Environment Agency and partners warn Berkshire river users to stay safe in heatwave

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.