How to Treat Stream Water on a Trail Run: Filters, Tablets and UV
Quick answer: Treat water from streams and lakes before you drink it. The CDC says to bring clear water to a rolling boil for 1 minute (3 minutes above 6,500 feet). Away from a stove, the options are a filter, chemical disinfection or UV light, and the CDC recommends filtering first and then disinfecting.
Why stream water needs treating
Clear, fast-flowing water can still carry parasites, bacteria and viruses. You cannot tell by looking. Treating it is part of planning any route where you expect to refill from natural sources.
Your options on the trail
| Method | What the CDC says it handles | Notes for runners |
|---|---|---|
| Boiling | Rolling boil for 1 minute (3 minutes above 6,500 feet) | The CDC's standard method |
| Filter, absolute pore size 1 micron or smaller | Removes parasites; not bacteria or viruses | Check the absolute pore size on the label |
| Filter, 0.3 micron | Removes bacteria and parasites; not viruses | Check the label for "absolute" pore size |
| Chlorine or iodine | Kill bacteria and viruses; do not work well against parasites | Iodine is not for people who are pregnant, have thyroid problems or iodine sensitivity, and no one should use it for more than a few weeks |
| Chlorine dioxide tablets | Kill Giardia; somewhat effective against Cryptosporidium | The CDC still advises filtering first |
| UV light | Kills parasites, bacteria and viruses in small amounts of clear water | Does not work well in cloudy water, so filter first |
A simple setup for trail runs
- Choose the clearest water you can. UV light does not work well in cloudy water.
- Filter first. This removes parasites and makes chemical or UV treatment work better.
- Then disinfect. Use chlorine dioxide tablets or a UV device, following the product instructions.
Plan refills before you leave
Treating water takes time, so plan where you will refill and how much you need between sources. The trail route planning guide shows how to map water on your route, and how much water to carry helps you size each stretch.
Carrying it
Front soft flasks are easy to refill at a stream, and a 2L hydration bladder adds capacity for long stretches between sources. Clean flasks and bladders after trips where you carried untreated water; see how to clean soft flasks and a bladder.
FAQ
Is a filter enough on its own?
Per the CDC, a 1 micron filter removes parasites and a 0.3 micron filter removes bacteria and parasites, but neither removes viruses. For full protection, filter and then disinfect.
Can I use UV on cloudy water?
UV works on small amounts of clear water. Particles in cloudy water block it, so filter first.
Source: CDC, water treatment options when hiking, camping or traveling.