Can I drink water from a stream if it looks clean?
Why Clear Water Isn't Safe
Pathogens are microscopic and invisible. A stream that looks pristine may be contaminated by animal feces upstream, even far from human activity. Giardia and Cryptosporidium are common in backcountry water and can cause severe diarrhea.
Bacteria like E. coli and viruses can also be present. The risk isn't just from visible pollution; it's from microorganisms that thrive in natural water sources.
- Giardia: a parasite causing gastrointestinal illness
- Cryptosporidium: chlorine-resistant parasite
- Bacteria: E. coli, Salmonella, etc.
- Viruses: norovirus, hepatitis A
How to Treat Water
The most reliable methods are boiling, filtering, and chemical treatment. Boiling for at least one minute (three minutes at high altitude) kills all pathogens. Filters with absolute pore size of 1 micron or less remove protozoa and bacteria but not viruses.
Chemical treatments like chlorine dioxide or iodine work against bacteria and viruses but may not kill Cryptosporidium. For complete protection, combine a filter with chemical treatment or use a purifier that handles viruses.
- Boil: 1 minute at sea level, 3 minutes above 6,500 ft
- Filter: 1 micron or smaller absolute pore size
- Chemicals: chlorine dioxide, iodine (not effective against Crypto)
- UV light: effective but requires clear water and batteries
Common mistakes
- Thinking that fast-moving or cold water is safe—pathogens survive in both.
- Relying only on a filter in areas where viruses are a concern (e.g., near human waste).
- Not treating water because it 'looks clean'—you can't see pathogens.
