Understanding Waterfalls

What is it that happens to water underneath waterfalls that can cause swimmers to get into trouble? Learn about sieves, syphons, flashy rivers and how bubbles rob swimmers of buoyancy.

Fatal accidents involving waterfalls are a tragedy during summer. What do you need to be aware of when you go swimming in waterfalls?

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. If you’re not experienced that’s the kind of thing that can make you panic, and panicking can get you into trouble.

If forced down by a waterfall it may be instinctive to try to swim against the current, back up thorough the bubbles, but you have to exit that stream of air filled fluid sideways and then swim to the surface.

Weight of water

In a waterfall you are placing yourself under a weight of water which can be on a scale from lightly massaging to bone crushing.

The power of the water varies with the flow rate, and when flow rates increase it gets heavier. Be aware of that: each time you get in you have to judge, how is it on this day, at this minute? When a river is in high flow, it is increasingly likely that rocks and tree debris could tip over a waterfall.

Siphons and sieves

Plunge pools tend to be carved out beneath waterfalls, and can make good swimming spots, ending in a rock lip.

At times of high flow, these pools can turn into torrents, and swimmers may find themselves forced again boulders or rocks at the pool exit, and held into it.

Water has a lot of power when it moves and one of the key hazards is a siphon. 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.

Flashy rivers have fluctuating hazards

Flashy rivers are ones where conditions change quickly with rainfall. There are various dangers associated with a flashy river, like those seen on Dartmoor and Snowdonia. A stream that was friendly and benign can, after a short period of heavy rainfall, rise by as much as 2-3ft and then have completely different risks and hazards. For example: a waterfall that is popular becomes dangerous, with swimmers more likely to be trapped against rocks or a fallen tree and held there by the strength and weight of the water.

Waterfalls often mean young rivers – which mean colder water

During the source to sea journey of a river, the water has a chance to warm up during warm and sunny periods. Waterfalls are often found at the rocky beginning of a river’s journey, and contain colder water than the more mature sections of the same river further downstream.

In mountainous areas you may still see patches of snow on north facing cliffs as late as May, and mountain stream water can be cold. People are caught out by cold mountain streams.

Risks of cold water include Cold Shock and Cold Incapacitation. See Risk of Cold Water. 


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 it’s 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.


  • The OSS believes that with the right to swim wild, comes the responsibility to look after yourself and do it safely. Pieces such as these are written in the spirit of one swimmer helping another develop their expertise – this is a developing area of knowledge and our safety knowledge is growing, not exhaustive. See the Swim Responsibility Statement.
  • The first version of this piece was written by Lynne Roper, an ex paramedic, wild swimmer. A posthumously printed collection of her swim stories is available: Wild Woman Swimming, £8.99, by The Selkie Press.