aerial view of a weir in the Thames

Understanding Weirs

Spot the dangers and avoid getting into trouble

Lawrence Hookham

What is it about weirs that can lead to swimmers getting into trouble, even to fatal accidents? Weirs and millraces are very popular spots for swimming, jumping and playing in water in summer. We explain the dangers, how to spot them, and what to avoid.

The clearest advice is to avoid weirs. Many people swim in the pools upstream from them, which is usually safe enough in low flows. Be careful not to slip and fall down the weir.

The big danger area is at the bottom of weirs, where stoppers, vertically circulating currents, can pull you back towards the weir and under the water. In some cases it’s impossible to escape. They can vary, according to the flow or conditions on the day, and the type of weir. Throw a floating stick in and watch what happens to it. If you see it heading back upstream and under – avoid. Some types of weirs, such as box weirs, are fatal to trapped swimmers and kayakers.

Weirs do vary, though, and many are popular places to play and swim in summer when water flows are lower. Check local advice and assess carefully, and if locals are safely playing in them, it might be safe to join in, but never assume this. Each time you are considering getting in you have to judge, particularly on that day and in that minute.

This advice also applies to millraces, where fast water rushes out in a stream from the mill or where there used to be a mill.

What else happens to water underneath or near weirs that can cause swimmers to get into trouble?

Bubbles mean lost buoyancy

If there is white water, if the water is foaming or disturbed by rocks or falling a distance, then you lose buoyancy. ‘White water is water mixed with air, and air bubbles have less density than water.  So you lose your buoyancy in it, are affected by gravity much more and may very well sink,’ explained Lynne Roper (swimmer and ex-paramedic, who wrote about the dangers of waterfalls). ‘If you’re not experienced that’s the kind of thing that can make you panic, and panicking can get you into trouble.’ It is instinctive to try to swim against the current, back up thorough the bubbles, but you have to exit that stream of air-filled fluid through the water around you and then surface.

Siphons and sieves: water has a lot of power when it moves

Water has a lot of power when it moves and one of the key hazards is a siphon. ‘Kayakers are the real experts in this,’ says Lynne. ‘But if there’s a gap between some rocks and water is forced through a smaller hold, then it’s the equivalent of a plug – a swimmer can get sucked in, plug the gap and then not move out again because they are pinned by the weight of the water.’ This is called a siphon.

‘A fallen tree or roots overhanging a bank can create the same effect, but this is called a sieve. In strong currents a swimmer can find they are pushed against roots or the bank, and the water holds them against it, and they can’t get away: the weight of the water holds them there.’

Detailed advice for paddlers on the wide range of water features, including weirs

Weight of water

In a weir you could be placing yourself under a weight of water, which in a way is dangerous. The height of a weir affects its strength.

‘The power of the water varies with the flow rate, and when flow rates increase it gets heavier, and can trap a swimmer against a rock or obstruction,’ says Lynne. ‘You have to be aware of that, and each time you get in you have to judge, particularly on that day and in that minute.’

Currents and eddies

Beware currents and eddies near weirs. Eddies – areas where water flows back upstream against the current – can send you at speed into the white water and a strong current.

High banks and exit points

It’s important to be aware of where the current is in a river. ‘In summer, with high spirits, teenagers are particularly likely to get into water that looks friendly, be moved along in it by the water and not be worried at all and then find they cannot get out because the bank is too high.’

Solutions to this include:

  • Working out exit points downstream before getting in
  • Getting into river pools where the water runs to shallow – so although you will go downstream, you will beach and get out
  • Swim upstream before you go downstream, where it’s possible to swim against the current

Colder than expected temperature causes cold shock

‘The biggest factor in people getting into trouble and drowning is the cold,’ says Lynne. ‘People think it’s hypothermia that’s the cause of that but you won’t get hypothermia for 30 minutes or probably longer in the warmer weather.’

Water in Britain is lower than 15 degrees much of the year. ‘At these temperatures you experience physiological responses when you get in including cold shock, the response where your body initiates a gasp reflex. If you are under the water or a waves comes over your head when this gasp reflex takes place you may inhale some water, and you don’t need to inhale much for it to cause serious problems with your breathing.  You don’t have to completely engulf your lungs for there to be a problem.’

After the initial gasp of cold shock comes a period of hyperventilation that you can’t control. ‘Breathing is really rapid and you can’t control it, which can lead to a feeling of panic,’ says Lynne. ‘New swimmers also tend to go upright in the water, in this position we are less buoyant, and then panic can really set in, increasing the chances of drowning.’

Getting in gradually and staying in depth until the cold water shock response has passed and breathing is under control is safer than jumping in.

In summer the water – or at least the top layer of water – can be much warmer, but it will still be cold deep down, so these warnings still apply. This is explained by ‘thermal layers’ or thermoclines

Cold swimmers are weaker swimmers (cold incapacitation)

Where water is cool compared to our body temperature, it eventually starts to cool swimmers down. ‘After 30 minutes or so our body’s response to cold is to restrict peripheral blood supply,’ says Lynne. ‘The blood supply in arms and legs and hands and feet goes much more towards the core, so even the strongest swimmers and fittest person loses strength and dexterity and the ability to coordinate their body.  Swimming becomes slower, ragged and short distances can take a long time to cover.

‘If the swimmer succumbs to early stages of hyperthermia they lose their fine motor skills and the ability to hold things in their hands.’ This can make it hard to get out, where it’s necessary to hold onto grass or tree roots with your hands in order to do so.


Fluctuating water levels mean that a popular jumping spot can become fatal if river levels have dropped, and currents can move underwater rocks: every episode of jumping needs its own risk assessment (going into the downstream section and exploring depth and underwater hazards). The fact that it’s a popular jumping spot or others have been seen jumping there before is no assurance of safety. The higher the jump, the deeper the water necessary beneath it.

See also Waterfalls: risks and hazards

Words Lynne Roper and Imogen Radford

two swimmers slide down a shallow weir Imogen Radford