A tarn is a small sheet of water cupped in high ground, and in northern Britain they sit in some of the finest walking country we have. Whether it is a shallow, reedy hollow on the fells or a dark, steep-sided pool beneath a crag, a tarn gives a walk a natural shape: you climb to it, you circle it, and you drop back down a different way. That circular element is what makes tarns so satisfying — and also what makes them worth planning properly. Upland ground is wetter, steeper and far more fragile than the paths most of us know from the valley.

Start with access, not with the route

Before you fall in love with a line on the map, check that you are allowed to be there. In England and Wales, much of the open fell is designated access land, and you have a right to walk and swim on it, but that right comes with seasonal and temporary restrictions. Lambing, shooting and forestry work can all close ground for a few weeks, and these closures change year to year. In Scotland, the access code is broader and more permissive, but the responsibilities are just as real: keep away from working farms, close gates behind you, and give livestock plenty of room.

Also look closely at the land between the tarn and your chosen car park. A tempting short cut across a private field can turn a pleasant day into an awkward conversation. Where a path leaves access land and crosses enclosed ground, it needs to be a right of way, not just a worn line in the grass.

Sort out the parking before you sort out the route

Upland car parks and verge laybys in the Lakes, the Dales, Snowdonia and the Southern Uplands fill early. On a summer Saturday or a bank holiday, the difference between arriving at nine and arriving at eleven is often the difference between walking from the intended spot and adding three miles of road walking to the start.

A few rules of thumb:

• Never park in a gateway, on a passing place, or on a verge that is visibly soft. Farm access matters more than your convenience.
• Use official pay-and-display sites where they exist, and keep change or a card handy.
• Note your exit point as carefully as your entry point. Coming off the fell after a swim, tired and in fading light, is not the moment to be improvising.
• If the nearest parking is full, walk in from further down the valley — it is usually a better day for it anyway.

Read the ground: paths, erosion and bog

Upland paths take a hammering. Where walkers spread out to avoid wet patches, the line widens, peat breaks up, and the scar gets worse each season. When a path is badly eroded, stay on the main line even if it means wet feet — a wide detour does more damage than a bit of mud. Where you see stone pitching, it was put there deliberately to hold the ground together; use it.

Expect bog. Even in a dry summer, the ground around a tarn holds water, and peat hags can be waist-deep. Gaiters and a pair of poles earn their place here. On steep ground, take the descent seriously: loose scree and wet grass are where most accidents happen, and a tarn circuit usually involves at least one steep pull in each direction.

Swimming in a tarn: cold, clear and worth thinking about

The water in a high tarn can be startlingly cold. Shallow pools may reach fifteen degrees or so in a good August, but deep, north-facing tarns can sit at ten or twelve degrees even in the middle of summer. Cold shock is the real risk, not the swim itself, so enter slowly, keep your breathing steady, and make your first dip a short one. Getting out before you feel cold is a skill worth learning.

Look before you leap. Outflows often feed a waterfall, submerged rocks sit just below the surface, and thick weed can trap a careless foot. In warm, still spells, blue-green algae can bloom and turn the water soupy; if it looks like pea green paint, do not swim in it. Cover any open cuts, and wash your hands before eating — weils disease is rare, but it is not a myth.

Build in time for photographs and a proper lunch

Tarns are photogenic in a way that is almost unfair: still mornings give you a mirror of the sky, and late afternoon light rakes across the water and picks out every ripple. Budget forty-five minutes to an hour around the water, and do not treat it as time wasted.

For lunch, find a spot on the leeward side, above the water rather than at its edge — midges gather along the shoreline, and sitting on bare rock rather than vegetation keeps your impact low. Take everything out with you, including fruit peel and crumbs that might seem harmless.

Weather, kit and honest timing

Check a fell-top forecast, not a valley one, and check it again in the morning. Cloud can swallow a tarn in twenty minutes and turn a straightforward circuit into a navigation exercise. Carry a paper map and compass, a head torch, a warm layer, a waterproof and a spare hat. Take more water than feels necessary, and remember that northern daylight is short between November and February. Tell someone where you are going and when you expect to be back. Then go and enjoy it — the tarn will still be there, and it will be better for the care you took getting to it.