Just as you can never step in the same river twice, as the Greek philosopher, Heraclitus, once said, nor can you swim in it twice. From a swimmer’s perspective, that’s both part of the joy of swimming in rivers and a source of hazards you don’t encounter when swimming elsewhere. Rivers are constantly moving and changing, which adds to the fun, but they need to be respected.
Before undertaking any outdoor swimming, familiarise yourself with our general safety advice. Swim with other people, avoid the consumption of alcohol, enter the water cautiously and ensure you can be seen by other water users. Then take heed of the additional hazards you find in rivers.
Check the flow
Where I regularly swim in the Thames, the current ranges from barely noticeable to a speed faster than any swimmer could match. Before any swim, as well as the water temperature, I check the flow. A visual check is the easiest way to start. Look for leaves or sticks drifting downstream. Pay attention as fast-flowing water can look deceptively calm. Other signs you can look for are water piling up in front of obstacles (e.g. bridge piers) and eddies and swirling water behind them. The current is typically stronger towards the middle of the river, so check both near your entry point and further away from the bank.
For my stretch of the river there are flow meters giving live measurements that are easy to look up online. From experience, I know roughly how the flow (given in cubic metres per second) relates to the speed of the water. For the navigable Thames, and some other rivers, you can also check on the Environment Agency’s website if there are any flow warnings.
A strong current doesn’t necessarily mean you can’t swim but it may change how you swim. See below for more on this.
Typically, and obviously, the more it rains, the faster the flow. However, keep in mind that some rivers drain large catchment areas. This means, for example, it could be raining in one part of the catchment area, but not where you are. If you are downstream of where the rain falls, you may experience increased flow without seeing any rain – see more on this in the section on weather below.
For rivers that flow into the sea, currents are further complicated by tides, which can affect the stream for a considerable distance inland. The Thames is tidal as far upstream as Teddington Lock, some 90 miles from the sea. In tidal rivers, the flow reverses direction roughly every six hours. Before attempting to swim, you should make yourself fully aware of the tide times and the impact on currents.
Many rivers are dammed either for water storage or for the generation of hydroelectric power. Downstream of dams, rivers may be subject to sudden changes in flow if there is a dam release, which can occur for a variety of water management reasons. Always check if you are swimming in a dammed river.
Natural changes in flow
As rivers wind their way downhill, they pass over different terrain. Sometimes they spread out into wide, calm pools. Other times they are squeezed into narrow channels or tumble over rocks. This is something you need to be aware of if doing a long-distance river swim, especially as rapids can be difficult to spot from the water. By the time you see them, it could be too late to escape their pull. Always do your research before a river swim to avoid being caught out.
Weirs and other river structures
Many rivers are controlled through weirs or channelled through man-made structures. Weirs in particular can be dangerous places. In some cases, the entire flow of a river is directed through a relatively small opening, which results in strong currents. Downstream of a weir the water is turbulent, with rotating currents that can drag you upstream back into a weir and hold you there. I know of one swimmer this happened to. He was able to swim strongly enough to escape the flow but it gave him a scare. Other people are not so lucky. Deaths at weirs are not uncommon. While they may look harmless in low flow conditions, weirs can be extremely dangerous. It’s best to avoid them. Locks are also dangerous. Not only can the opening of lock gates cause dangerous currents, they can be narrow and busy with boat traffic. Swimming is usually prohibited near locks, but I’ve seen plenty of people doing it. Please don’t be one of them.
Dealing with currents
Even a gentle current can be difficult to swim against. If you have any doubts about your ability to swim faster than the current, then I’d recommend a one-way down-river swim where you aim for a known and safe exit point. When using the same entry and exit point, best practice is to swim upstream first while you’re fresh and return with the current. Remember the current is usually stronger in the middle than at the edges so if you’re struggling to make headway going upstream, tuck into the riverbank. Also, the flow tends to be stronger on the outside of a bend than the inside, which may influence which side of the river you swim on (but see also the section on other river users). Swimming in fast-flowing rivers is fun and can be done safely, but you need to be alert to the risks. Give yourself plenty of time to approach the bank with your exit point as you head downstream as if you miss it, you may not be able to swim back. It can be useful to have a second exit point further downstream, in case you miss the first.
Closely related to changing flow rates in rivers is the potential for changes in depth. In tidal sections, these depth changes occur twice daily and can be huge. The River Severn is famous for its tidal range, which can be as much as 15m change in water level in six hours, and the Severn Bore, a wave travelling upstream created by the incoming tide. Downstream of Teddington Lock on the Thames, the water level varies by several metres with each tide, yet upstream the level is almost always constant as it’s controlled by the sluice gates at the weir. However, it is still liable to flooding in extreme conditions. On other rivers, dam releases and rainfall can both cause sudden changes in depth as well as flow.
Changes in depth may affect how easy it is to enter and exit the water. This is important if you are expecting the depth to change while you are swimming. You should ensure your exit point will be available to you when you finish swimming and not several metres out of reach.
Rivers also change depth along their length and, if they are not managed, with time. Sometimes these changes are somewhat predictable. Currents tend to sweep faster around the outside of a bend and slow down on the inside. This often results in shallow water on the inside of a bend and deeper water on the outside. When swimming downstream, you might be tempted to “cut the corner” through the inside of the bend. However, this may be slower than sticking with the faster current around the outside and you may find yourself in water too shallow for swimming.
Other river users
You’re not the only one who enjoys rivers. You may encounter anglers, kayakers, rowers and powered boat users among others, with the danger ranging from a nuisance (e.g. getting caught in a fishing line) to deadly (e.g. being struck by a fast-moving boat).
There is no consensus on whether swimmers should swim on the left of the river, facing oncoming traffic, or on the right, swimming on the same side as regular river traffic (there are, however, some strong opinions on each side of the debate). I’ve seen both, and both have pros and cons. If local swimmers have already adopted one side or the other, it’s best to stick with that as regular river users will know to expect you there. Otherwise you will need to decide on a case-by-case basis. For example, if you are swimming upstream and then returning to your starting point, it may be better to stick to the same bank rather than cross the river as crossing may be more dangerous than swimming on the ‘wrong’ side. If you only have access to one bank of the river, it may be best to stick to that side in case you need to get out. Alternatively, you may find that the side of the river with public access is used by anglers. In this case, it may be better to swim on the other side, regardless of which direction you’re swimming.
It's essential that you give other water users every opportunity to see you. A brightly coloured hat is a good start. Using a tow float is better. They can be spotted from a much greater distance than a swimming cap. In poor light, you can add a light to a tow float. Either a waterproof light fitted to the outside or a bike light inside a tow bag work well.
You need to check around you frequently – not just looking forward to see where you’re going, but sideways and behind too. A rower can bear down on you silently and at high speed. Even if you use a tow float, you are still difficult to see in the water to someone travelling backwards. If you can persuade someone to kayak or paddleboard next to you, they can be an extra pair of eyes on the water for you, plus they will help other river users see you.
It won’t do you any harm to clip an emergency whistle to your tow float. If someone hasn’t seen you, they may hear you. Plus, if you get into difficulties in another way, it’s a good way to attract attention.
I’ve heard several reports of swans threatening and even attacking swimmers. They are probably the most likely wildlife hazard for river swimmers in the UK. This doesn’t mean all swans are dangerous. There’s a pair on the river where I regularly swim that are curious but not threatening (at least not yet). Swans usually make it pretty clear if they don’t want you to approach and some are known to be territorial.
I’ve seen seals a few times in the Thames. Once I saw one tearing large fish to pieces in a matter of seconds. They clearly have powerful jaws. Another time, one surfaced right behind a friend of mine, and then disappeared into the murky water. Knowing you’re sharing the water with something several times your weight and perfectly adapted to swimming and hunting in the water is unnerving. Luckily seals rarely deliberately attack people. If you know they are around, it’s best to stay away and observe them from a distance. They are known to approach people, and they use their jaws to explore their environment, so occasionally grasp limbs or fins. In the unlikely event that a bite causes an injury, seek medical assistance as you will probably need antibiotics.
The only other creature I’ve heard of that may bite a swimmer in UK rivers is a pike. Apparently, they are attracted to bright flashing objects. In the one case I heard of, the swimmer was wearing a silver watch.
Not wildlife, but related, one swimmer told me they were once bitten by a dog that jumped into the water next to them!
When travelling, always check local advice before entering rivers. Around the world you can encounter anything from poisonous snakes to crocodilians and even sharks.
Because you have the potential to swim faster in rivers if you’re being carried by the current, the risk of injuring yourself in a collision increases. This is another good reason to keep a sharp lookout while swimming or ask a kayaker to paddle next to you. In very fast flowing water, it’s possible to get pinned against obstacles. Stay away from tree branches on the waterline too as the force of water can push down if you get stuck against one.
Rivers can carry a lot of debris, both natural and man-made. Leaves and small sticks mostly won’t hurt you (although getting a leaf plastered across your goggles or mouth can be a little panic-inducing). Larger sticks or pieces of litter can cause cuts and grazes. Things I’ve swum into include boat fenders, bits of wood with nails in, an armchair, a coconut, a potted plant and an onion. Debris usually increases after stormy weather or heavy rain and in the autumn.
Note, like people, large pieces of debris can get trapped in turbulent water below a weir. If you swim here, not only do you risk getting and stuck and drowning, you may also be battered by branches.
While water companies are allowed to discharge sewage into rivers under extreme circumstances, evidence collected by environmental groups suggests these discharges are more common than they should be. On top of that, rivers can be polluted by runoff from farms and industrial discharges. As rivers are constantly moving, testing the water quality can never give you 100% reassurance, only guidance. There is only one recognised stretch of river that has Bathing Water Status in the UK, and which is therefore monitored regularly – however, this is only in the summer. You therefore have to look elsewhere for data and information. A good resource is this map created by the Rivers Trust (https://www.theriverstrust.org/key-issues/sewage-in-rivers). Some sewage treatment works also share live data on when they are discharging, so these are worth monitoring. Local swimmers can provide advice. Ultimately though, you need to trust your own instincts and judgement. If it looks or smells bad, stay out. Water quality tends to deteriorate after heavy rain that may result in both agricultural runoff and sewage spills.
Rivers can be extremely sensitive to the weather. Years ago, I took part in a kayaking slalom event on some rapids. In the morning the course was easy to navigate. By the afternoon, it was transformed into a raging torrent because of a rainstorm further upriver. As well as paying attention to local weather conditions, you should therefore always check the weather upstream of your swimming spot. Remember too that there is a time lag between when rain falls on the ground and when it reaches the river. This delay is unique to each river depending on a range of factors including the size of the catchment area to the type of ground the rain falls on. In general, the further upstream you are, the quicker the current changes with rainfall but beware as rivers can change from benign to deadly in a matter of hours.
Watch out for wind too. Not only will a strong wind cool you more quickly while swimming and changing, it can make the river conditions more challenging. It can also make the current look like it’s flowing in the opposite direction, which might prompt you to swim in the wrong direction first. The water surface can get particularly choppy and challenging to swim in when the wind is blowing in the opposite direction to the current.
Finally, remember your river needs looking after too
Rivers are wonderful places to swim and explore but they can also be vulnerable to damage by us. Please protect your river. Take care entering and exiting the water to ensure you don’t damage the banks. Make sure all your kit has been cleaned and dried since previous use to ensure you don’t transfer invasive species. Observe the wildlife, but don’t approach and disturb it, and be gentle with plants.