Why an Upland Tarn Is a Different Beast

A lowland lake in July might sit at 18°C or 19°C, pleasant enough for a long, lazy swim. A tarn at 500 or 600 metres in the Lake District is another matter entirely. Even in a good summer, the surface layer might reach 14°C while a metre below it stays closer to 9°C or 10°C. In May, or after a spell of north-westerlies, you can be looking at 6°C or 7°C.

The reason is simple: altitude, depth and exposure. Upland tarns are shallow enough to be chilled by night-time air, deep enough to hold a reservoir of cold water that never quite warms, and perched in bowls where wind strips heat off the surface. They are also often a long way from the car, which changes the consequences of getting cold. Swimming in them is one of the great joys of the fells — but it rewards a bit of thought before you pull your boots off.

Understanding Cold Shock

Cold shock is not the same as hypothermia. It is an automatic physiological response to sudden skin cooling, and it happens in the first thirty seconds of immersion. You gasp, you hyperventilate, your heart rate and blood pressure spike. That gasp is the dangerous part: if your head goes under at that moment, you can inhale water.

It affects everyone. Being a strong swimmer, being young, being fit, being used to the sea — none of it switches the reflex off. What experience gives you is the knowledge that it will pass, and the discipline not to fight it.

• First 30 seconds: gasping and rapid breathing. Keep your head up and stay in your depth.
• One to three minutes: breathing settles. This is the window to start swimming properly.
• Five to ten minutes: cold incapacitation begins. Grip weakens, strokes get sloppy, and thinking slows before you notice it.

For most fell tarn dips, five to ten minutes in the water is plenty. This is not the place for a mile-long swim.

Entering Gradually, and Breathing Through It

Walk in. Resist the urge to dive, jump or cannonball in — it triggers the gasp reflex at its worst and removes your control over the entry. Wade to knee depth, splash a little water on your wrists and the back of your neck, then spend thirty seconds just standing there, breathing out slowly and deliberately. Long exhales, not short panicky ones.

Once your breathing has settled, lower yourself in and keep your head above water for the first minute or two. Only put your face in when you feel genuinely calm. A tow float is worth carrying on bigger tarns — it makes you visible to anyone else on the water and gives you something to rest on if you need a moment.

Watch the bottom as you go. Tarn edges are often silty, and the drop-off can be sudden. Submerged boulders are slippery with algae. Take your time and keep your footing.

Know Your Exit Before You Enter

This is the single most useful habit in cold water. Stand on the shore, look at the water, and pick the exact spot where you will get out. Then ask whether you could reach it if your hands were numb and your arms were tiring.

The reason is that cold water and wind do not care about your original plan. Wind pushes you along the surface. An offshore breeze will move you away from the bank faster than you expect, and the shore you launched from may be the hardest one to climb out onto. Look for:

• A gently shelving bank rather than a steep rocky edge or an undercut peat lip.
• A spot with handholds or a shallow shelf you can crawl onto if needed.
• A section of shore that is not in the direct line of the wind.
• A backup exit in case you drift.

Stay close to shore throughout. Ten metres out along a shelving bank gives you far more options than fifty metres out in the middle of the tarn.

Never Swim Alone

Swim with someone who is watching from the bank or, better, in the water with you. A companion on shore should know exactly how long you plan to be in, and should be ready to call you out — not to ask politely, but to tell you. Cold swimmers are notoriously bad at judging their own state, and the first sign of trouble is often someone insisting they are fine.

Tell someone at home where you are walking and when you expect to be back. Phone signal in the fells is patchy at best, so do not rely on being able to call for help. Carry a means of signalling, know your grid reference, and remember that mountain rescue teams are volunteers who would rather be called out for a false alarm than a body recovery.

Also check the water itself before you get in. In late summer, some tarns develop blue-green algal blooms, which look like green paint or pea soup along the shore. If you see it, do not swim, and keep dogs out too.

Getting Out and Warming Up Properly

Get out while you are still enjoying it. Fingers that are starting to claw, a slurring of speech, or a sudden feeling of heaviness are all signals to leave immediately, not in five minutes. Dress fast and in the right order: towel, then every dry layer you have, then hat and gloves, then something windproof. A flask of hot, sweet tea and a bag of jelly babies will do more for you than any expensive kit.

Do not jump straight back into a long descent. Sit, shiver, drink, and let your core temperature come back up before you put your boots on. Shivering is a good sign — it means your body is doing its job. If shivering stops and you feel strangely warm and sleepy, that is not recovery, and you need to get off the hill and into shelter quickly.

Done sensibly, a dip in a cold tarn is one of the finest ways to finish a day on the fells. Enter slowly, stay shallow, keep a friend close, and plan your exit before your feet touch the water.