Safe lake swimming: the complete guide

A lake looks like the easy option. There's no tide, and often there's a shelving bit of shore where you can wade in at your own pace. Most of the time that's exactly what you get. But lakes have their own ways of catching swimmers out, and nearly all of them sit below the surface or at the edges: cold water under a warm top layer, a pull near pipes and outlets, algae in warm weather, and banks that are easier to get down than to climb back up.

This guide covers what to check before a lake swim, what current UK guidance says about each point, and where to read more. Some checks are specific to lakes: depth, size, inlets, outlets and blue-green algae. Others apply to every swim: getting in and out, permission, other water users, lifeguards, phone signal and the forecast.

If any answer points to danger, don't swim. If more than two call for caution, take extra care or choose not to swim. Every body of water is different, so treat the questions as a guide. The judgement is yours.

Getting in and getting out

Not knowing your way out is a danger, so sort it first. Plenty of lakes are easy to get into. The harder part is getting out at the same spot when you're cold and tired, and having a second option if you can't.

The RLSS advises planning your exit options before you get in, with several along your route in case you need to stop early. It also asks you to look at the ground: is the shore rocky, flat or slippery? Some lochs, like some stretches of river and remote coast, have steep or undercut banks with few safe places to climb out. RoSPA warns that water is often deeper than it looks, and that swimmers can find themselves trapped and unable to get out.

Former quarries and clay pits are the extreme case. RoSPA notes that their side slopes can change suddenly, and the Outdoor Swimming Society (OSS) warns that stone quarries can drop sharply away from shallow beaches.

Two habits help. Swim along the bank rather than straight across, which the OSS recommends so you can get out as soon as you feel cold. And think about the wind: the OSS points out that if you swim out with it behind you, you'll be swimming back into it.

Depth and size: the cold underneath

Ask about size the same way, from tennis-court sized up to way bigger than a football pitch. Take care in a large lake for the same reason.

The physics behind it is thermal stratification. In summer the sun and air warm the top of a lake. In a lake that's deep enough, that warm water sits on a colder layer the wind can't mix in, with a sharp change in temperature between the two called the thermocline. The cold layer stays cut off from the air until autumn cooling mixes the lake again. The OSS puts the water in the depths at around 4 to 7°C.

The RLSS advises keeping away from deeper water, which will be colder. Jumping in is a bad idea for the same reason: you can go straight through the warm layer. There's more in why big, deep lakes stay cold below the surface.

Wild estimates each lake's water temperature from published models and shows it as a band, such as cool or cold, rather than an exact reading. Treat it as a guide to the water near the surface, not to what's underneath.

Small, shallow lakes

The other end of the scale brings a different risk. The smaller and shallower a lake is, the greater the chance of poor water quality, such as blue-green algae blooms.

Shallow lakes don't have the depth to keep a cold layer. A review of lake stratification by Boehrer and Schultze describes how they're mixed from top to bottom by the wind, and notes that smaller lakes tend to gain heat faster in spring and, in hot spells, peak a degree or two higher. That's lovely for swimming. But temperature is also one of the conditions Natural Resources Wales lists for blue-green algae blooms, alongside phosphorus and light, and DAERA points to sunlight, still or slow-flowing water and nutrients. Read more in small, shallow lakes.

Inlets and outlets

The OSS notes that spring- or stream-fed quarries often have a cooler patch where the water comes in. Find out where those are before you swim through one.

Outlets are the other side of the same question: drains, overflows and natural outlets to rivers. They change the flow of water and can draw you closer, so take care around them too. Water companies describe the same thing at reservoirs. Yorkshire Water warns of strong undercurrents where water is continually drawn into large submerged pipes, and the OSS advises keeping a wide distance from dam towers and spillways. The detail is in inlets and outlets.

Blue-green algae

Blue-green algae (cyanobacteria) can produce toxins. The Environment Agency says you can't tell whether a bloom is toxic just by looking, so assume it is, keep pets and children away, and avoid skin contact with the water or the algae. Affected water can look less clear and turn green, blue-green or greenish-brown, and scums can look like paint, mousse or small clumps.

The Environment Agency and UKHSA list skin rashes, eye irritation, vomiting, diarrhoea, fever, and muscle and joint pain among the symptoms after contact. The toxins can kill pets. In Northern Ireland, DAERA says blooms are most common from May through to September, but they can occur at any time of year.

In England, report a bloom to the Environment Agency on 0800 80 70 60, which is a 24-hour line. There's a fuller guide, including dogs and the numbers for the rest of the UK, in blue-green algae.

Reservoirs and quarries

Most reservoirs don't allow swimming. Yorkshire Water says its reservoirs stay at a very low temperature year-round, that machinery and strong currents sit beneath the surface, and that blue-green algae can be a problem. United Utilities adds deep water and steep sides that make it hard to get out, and remote sites where emergency services can struggle to reach you.

Flooded quarries can be cold, deep and steep-sided, with underwater hazards. The OSS also notes that some have been turned into managed open water lakes. The RLSS recommends lifeguarded, recognised venues. More in reservoirs and quarries.

Permission and access

The point is practical as much as legal. Some bodies of water are restricted for good reasons, so find out why before you swim.

On the legal side, the OSS explains that in Scotland swimmers have a statutory right of responsible access to most land and inland water. In England and Wales the law is less clear and more restrictive. Getting into the water across private land, other than from a public footpath or road, is technically trespass, which is a civil wrong rather than a crime in most circumstances. If you're asked to leave, the OSS advice is to stay polite and go at a sensible pace.

The OSS's Outdoor Swimmer's Code adds the courtesy side: research the location, use recognised paths, respect landowners and neighbours, and park without blocking gates or emergency access.

Other people on the water

RoSPA warns that swimmers can be struck by boats and other craft, and recommends a brightly coloured hat and a tow float so you're easier to see. The RLSS says to always use a tow float when boats or other watercraft are about. The OSS asks swimmers to share the water considerately with anglers, boaters and paddlers.

Check too whether there's a designated swim area, a designated swim time or lifeguards on duty. The RLSS advice is to choose a lifeguarded, recognised venue where you can. See lifeguards, flags and designated swim areas.

Forecast, signal and someone who knows

The RLSS says to check current conditions and the forecast, and warns that storms, heavy rain and strong winds can make swimming dangerous. Wild's spot pages carry today's weather and estimated water temperature, a 7-day forecast and recent reports from swimmers who've been there.

Check for phone signal too, because without it, how would you raise the alarm? The RLSS notes that coverage can be poor, so check you have a signal and keep your phone in a waterproof case. The OSS adds that remote quarries may have no signal at all.

Before you go, tell someone where you're swimming and when you expect to be back. The RLSS suggests being as specific as you can, and letting your contact know once you're out.

Cold water, and if it goes wrong

Whatever the season, think about the cold. RoSPA describes cold water shock as a response to entering water at 15°C or lower that can leave you struggling to breathe and to swim. Get in slowly, and read cold water shock and hypothermia before your first cold swim.

If you get into trouble, Float to Live: lie back, control your breathing, then think about getting out. The breathing tip of in for three, out for six is in panic in the water.

If you see someone else in trouble at a lake in the UK, the RLSS says to call 999 and ask for the Fire Service, look for rescue equipment, throw something that floats, and not to go in yourself. There's more in who to call.

Sources

1. Royal Life Saving Society UK, Top Recommendations for Safer Open Water Swimming
2. Royal Life Saving Society UK, Open Water Swimming Venues and Safety
3. RoSPA, Open water swimming
4. RoSPA, Lake and pond safety
5. Outdoor Swimming Society, Understanding Thermoclines
6. Outdoor Swimming Society, Understanding Flooded Quarries
7. Outdoor Swimming Society, Understanding Reservoirs
8. Outdoor Swimming Society, OSS Intermediate Tips for Safe Summer Swimming
9. Outdoor Swimming Society, Is It Legal?
10. Outdoor Swimming Society, The Outdoor Swimmer's Code
11. Reviews of Geophysics (Boehrer and Schultze, 2008), Stratification of lakes
12. Environment Agency, Algal blooms: advice for the public and landowners
13. Environment Agency and UK Health Security Agency, Swim healthy
14. Natural Resources Wales, Blue-green algae
15. Department of Agriculture, Environment and Rural Affairs (Northern Ireland), Blue-Green Algae
16. Yorkshire Water, Visitor safety
17. United Utilities, Stay safe at our reservoirs

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.