When the Sun Comes Out, So Do the Swimmers
There’s something almost irresistible about a hot summer’s day and an inviting stretch of open water. The moment temperatures nudge past 25°C in the UK, rivers, lakes, lidos, and reservoirs suddenly become the most appealing places on earth. Wild swimming — once considered a niche pursuit for hardy adventurers — has become a mainstream summer ritual for thousands of people across the country.
And honestly, who can blame anyone for wanting to cool off in a sparkling river or a tucked-away swimming hole? There are few better antidotes to a sweltering afternoon. But with a surge in popularity comes a genuine need for awareness. Hot weather doesn’t just bring swimmers to the water — it also brings a range of conditions that can affect how safe that water actually is.
Why Hot Weather Changes the Water Quality Picture
It’s easy to assume that a beautifully clear, sun-dappled lake is automatically safe to swim in. Unfortunately, appearances can be deeply misleading, particularly during hot spells. Warm temperatures have a significant knock-on effect on the biological and chemical composition of natural water bodies, and not always in a good way.
The Rise of Blue-Green Algae
One of the most well-known warm-weather hazards is blue-green algae (cyanobacteria). When temperatures rise and water becomes warm and still, these microscopic organisms can multiply rapidly, forming blooms on the surface. Some strains produce toxins that are harmful to humans and animals alike, causing skin rashes, stomach upsets, and in serious cases, more severe health effects.
Blue-green algae blooms can appear almost overnight, particularly in slow-moving or still water such as lakes and reservoirs. They often look like green paint, foam, or a pea-green scum on the surface. If you spot anything suspicious, it’s best to stay out entirely — and keep dogs well away too.
Increased Bacterial Activity
Warm water also accelerates the growth of bacteria. In rivers and coastal areas, E. coli and enterococcus levels can spike during hot periods, especially after heavy rainfall washes agricultural runoff, animal waste, and urban pollution into waterways. Ironically, the long dry spells that often accompany hot summers can also concentrate pollutants in reduced water volumes, raising bacterial counts in rivers that might otherwise be relatively clean.
More People, More Pressure
There’s also a human factor to consider. As more swimmers flock to popular spots, the cumulative impact on water quality increases. Sunscreen, insect repellent, and other personal care products enter the water, and in heavily used spots, human waste can become a concern if facilities are inadequate. It’s worth thinking carefully about lesser-known, quieter locations where the water is under less pressure.
How to Check Before You Swim
The good news is that you don’t have to guess. There are practical steps every wild swimmer can take to make a more informed decision before getting in the water.
- Check local authority alerts: The Environment Agency publishes bathing water quality data for designated swimming spots in England. These ratings are updated regularly throughout the season.
- Look for algae bloom warnings: Local councils, wildlife trusts, and swimming groups often issue warnings on social media when blooms are detected. Follow relevant groups for your area.
- Use a water quality test kit: For spots that aren’t officially monitored — which is the vast majority of wild swimming locations in the UK — testing the water yourself is one of the most reliable ways to assess safety. A simple test can give you a clear picture of bacterial contamination levels before you take the plunge.
- Trust your senses, but don’t rely on them alone: Cloudy water, unusual smells, dead fish, or surface scum are all warning signs. But clean-looking water isn’t a guarantee of safety either.
Getting the Most from Your Summer Swimming
Hot weather wild swimming is one of life’s genuine pleasures, and we’re absolutely not here to put you off. The aim is simply to swim smarter, so you can enjoy it more — and more often.
Going out early in the morning is a great habit to get into. Water tends to be calmer, there are fewer people about, and algae blooms are often less visible on the surface before the sun has had a chance to warm things up. Early-morning swims also have a wonderful quality of light that afternoon dips simply can’t match.
It’s also worth building a relationship with the specific water bodies you love to swim in. Learn their seasonal rhythms. Notice when the colour changes, when the current picks up after rain, and when they feel different underfoot. Wild swimmers who know their spots intimately are far better placed to make sound judgements about when it’s safe to swim and when it’s better to wait.
Test the Water You Swim In
If you’re serious about wild swimming — and we suspect you are — testing your water regularly is one of the smartest habits you can adopt. It takes just a few minutes and gives you real, actionable information rather than guesswork. Whether you’re a regular at the same river each week or an explorer who loves discovering new spots, a test kit belongs in your swim bag alongside your tow float and neoprene gloves.
Browse our full range of water quality test kits, designed specifically for wild swimmers and open-water enthusiasts, at wildswimmingtests.co.uk/shop. Swim informed, swim safe, and make the very most of the warm weather while it lasts.
📷 Photo by Jaz Blakeston-Petch on Unsplash
