The human body can go several days without food, but it never forgives prolonged dehydration, especially under exertion and at altitude. Losing more than 2% of your body weight in water is enough to send physical performance and mental sharpness plummeting — a dangerous combination in the middle of a technical descent. Water management isn't just another logistical detail: it's a skill in its own right, blending terrain reading, needs calculation, and the technical choice of treatment.
Never trust a stream's clarity alone. Crystal-clear water can carry bacteria (E. coli, salmonella), viruses, and protozoa such as Giardia duodenalis or Cryptosporidium — invisible to the naked eye and utterly indifferent to the beauty of the landscape around them.
Reading the terrain before filling your bottle
A few simple markers let you assess the risk before you even reach for your filter:
- What lies upstream. A grazed alpine pasture, a shepherd's hut, or even a simple transhumance path is enough to contaminate an entire drainage network downstream, sometimes for several kilometers. Check the topographic map to spot summer grazing areas above your collection point.
- Flowing water versus stagnant water. A torrent tumbling over rocks stays oxygenated and continuously dilutes pathogens; a pond, a shallow lake, or a puddle at the bottom of a valley concentrates them instead, while also encouraging algae growth. Given sources of comparable quality, always favor moving water over still water.
- The altitude of the source. The closer you get to the origin — a tapped spring, a resurgence, a melting snowpatch — the less chance the water has had to pass through pastures or inhabited areas. Water drawn just below a summit is statistically safer than water drawn at the bottom of a farmed valley, though that never excuses skipping treatment.
How much to carry, how much to drink
Needs vary considerably with exertion, temperature, and altitude, but a few benchmarks help with planning a day out: in mild weather with moderate effort, plan on roughly 2.5 to 3 liters a day; in intense heat, at high altitude, or on sustained elevation gain, that figure can climb to 4 or even 6 liters. It's better to sip small amounts regularly throughout the hike than to drink large quantities all at once, and keeping an eye on urine color remains the simplest indicator of good hydration. Always keep a safety reserve of half a liter that you only touch if the route falls behind schedule or if the next source marked on the map turns out to be dry — which happens often in late summer, even for small streams shown in blue.
Comparing treatment techniques
There's no single best method in absolute terms, only trade-offs between speed, weight, autonomy, and the range of protection offered.
| Method | Treatment time | Weight | Effectiveness against viruses | Effectiveness against bacteria / protozoa |
|---|---|---|---|---|
| Hollow-fiber filter (Sawyer, Katadyn) | Near-instant, continuous flow | 60 to 150 g | No — pores too large | Excellent (mechanical removal) |
| Chemical (chlorine, chlorine dioxide) | 30 min to 4h depending on dose and water temperature | 20 to 30 g | Good to excellent | Good to excellent |
| Boiling | 1 min at a rolling boil, plus heating time | Weight of the stove already carried | Total | Total |
| UV (SteriPEN-type pen) | About 90 seconds per liter | 100 to 150 g with batteries | Good in clear water | Good, but ineffective in cloudy water |
The filter remains the most practical everyday choice in Europe, where viral risk is marginal. Chemical tablets make an excellent backup in case of filter failure, despite their taste. Boiling remains the gold standard when doubt is at its highest, but its fuel cost limits it to situations that truly warrant it. UV treatment needs already-clear water to be fully effective, so it's often paired with a pre-filter.
"Water that looks pure has never stopped anyone from catching Giardia — treatment makes the difference, never appearance."
Giardia: recognizing the symptoms
Giardiasis deserves special attention: it's the most commonly reported contamination after trekking in European mountain ranges. The responsible protozoan has a long, deceptive incubation period — the first symptoms generally appear one to three weeks after ingestion, well after most hikers have finished their trek, which often delays diagnosis. Typical signs include persistent diarrhea, bloating, foul-smelling gas, abdominal cramps, and unusual fatigue. Left untreated, the episode can drag on for several weeks. If these symptoms appear within a month of a trek, it's worth mentioning it explicitly to a doctor: giardiasis responds well to treatment once identified, but it's often mistaken for a simple passing stomach bug.
Planning ahead with the map
Before setting out, locate marked springs, stable river sections, and staffed mountain huts on your topographic map — these generally serve as reliable resupply points throughout the season. Stay alert in late summer: many streams shown in blue dry up completely, and the information isn't always kept up to date. The most reliable approach is still to ask hut wardens or local shepherds directly, since they know the real state of the water network far better than any map.