Why a clear Highland burn isn't automatically clean

Water running cold and peaty off a Welsh hillside looks about as pure as anything on earth. It isn't. Pathogens reach upland water from sheep, deer, cattle, wild birds, rats and other walkers, and they don't announce themselves. The usual suspects include Campylobacter and E. coli (bacteria), Giardia and Cryptosporidium (protozoa), plus leptospirosis from rodent urine, which can enter through a cut or a splash to the eyes and mouth. A single mouthful from the wrong trickle above a sheep graze can knock you flat for a week.

The practical rule is simple: treat everything, wherever you are and however good it looks. There is no reliable way to judge a source by eye. Clear water can be contaminated; brown peaty water is often perfectly drinkable once treated. Risk does rise after heavy rain, when muck washes off the hillsides, and in popular areas where human waste is buried too close to watercourses.

Choosing a source: springs, streams and upland tarns

Source selection cuts your risk before you've even got the filter out.

• Best bets: fast-flowing water above the last grazing fence, or a spring emerging directly from rock or scree. Small, cold, tumbling tributaries beat big, slow, lowland rivers every time.
• Avoid: standing water, anything warm and stagnant in late summer, water with green or blue-green surface scum (cyanobacteria produce toxins that boiling and filtering will not remove), and anything immediately downstream of a dead animal.
• Tarns: magnificent for a swim, poor for drinking. They are stagnant, heavily used by birds and often full of silt. If a tarn is your only option, take water from an inflow stream or trickle feeding it rather than the tarn itself, and skim from the surface rather than scooping bottom sediment.
• Bothy supplies: many bothies have a nearby burn or spring, but where a tap or butt is fed by roof catchment, expect leaf litter and bird droppings in the mix. Treat it.

Boiling: the method that never fails

If you have fuel, boiling is the gold standard. Bring the water to a rolling boil and hold it for one minute — three minutes if you're above 2,000 metres, which is rare in the UK but worth remembering on Alpine trips. That kills bacteria, viruses and protozoa, including Cryptosporidium, which shrugs off chlorine-based treatments entirely.

The drawbacks are practical rather than technical. Boiling costs fuel, takes time, and gives you hot water when you probably wanted cold. It also does nothing about chemical contamination, and boiled water tastes flat — pour it between two bottles a few times to put the oxygen back. On a bothy evening, though, a pan of boiled and cooled water covers your drinking, cooking and washing needs in one go.

Chemical treatment: light, slow and full of caveats

Chlorine dioxide tablets are the best of the chemical options. They deal with bacteria and viruses after around 30 minutes, but Cryptosporidium needs a much longer contact time — up to four hours, and longer still in near-freezing water. Read the instructions properly and then double the time in winter. Iodine works but tastes foul and isn't suitable for anyone pregnant or with thyroid problems. Sediment and cold both reduce effectiveness, so pre-filter cloudy water through a cloth or buff first, and never sip early "just to test it".

Filters and purifiers: the day-walk workhorse

Hollow-fibre filters rated at 0.2 microns or finer physically strain out bacteria and protozoa, which are far larger than that, while letting water through. They are fast, lightweight and taste-free. Squeeze filters suit solo walkers, gravity bags are brilliant for a bothy full of people, and pump filters handle silty tarns without clogging as quickly — at the cost of weight and effort. Most filters will not remove viruses, which are small enough to pass straight through. Viruses are uncommon in UK upland water, but in lowland or heavily used areas a filter followed by a chemical treatment is the belt-and-braces approach.

Two rules keep a filter alive. Pre-filter muddy water or let it settle first, or you'll spend the evening backflushing. And never let a hollow-fibre filter freeze — ice crystals rupture the fibres and the filter fails silently, with no visible sign. Sleep with it in your bag on winter trips. UV purifiers work well on clear water but need batteries and are defeated by peat-stained tarns.

Practical habits that make it easy

• Carry at least two litres of capacity and drink steadily rather than gulping at the end of the day.
• Keep "dirty" and "clean" bottles separate and know which is which — a strip of tape or a different colour cap does the job.
• Treat a full bottle at once and set an alarm for the contact time, so you're never guessing what's safe.
• Fill up at good sources when you pass them, even if you're not thirsty yet.
• Don't assume a bothy will have usable water. Carry what you need for the evening and morning.
• Pack a backup method. If you're already dehydrated with a stomach upset, you don't want to discover your only filter is blocked.

Get into these habits and water stops being a worry. You fill up at a promising trickle, treat it while you eat your sandwiches, and walk on with a litre of cold, clean water that costs you nothing but a few minutes and a little kit discipline.