I was just reading this question What is the safest way to purify water?

Now I walk a lot in Snowdonia in North Wales and on occasion I drink from mountain streams. I've even camped next to mountain streams and used the water as my main water source.

I haven't used any purification tablets or boiling (generally) and suffered no ill effects. I always ensure the water is fast running and 90% of the time it's fed from mountain springs, so I've always assumed it's safe, in fact I'm sure I've read it's generally cleaner than most tap water!

I'm not sure where I picked up this information, I'm pretty sure my Dad used to do this.

There will be very limited chemical pollutants (from fertilizers, etc.) in the water as the area has virtually no arable farming.

Mining is probably the worst potential pollutant but I always check the source of the water(I've been walking in the are since I was very young and know the sources of most of the streams)

Am I mad or risking my health?

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    It would be nice if someone wound up some numbers. In this regard, IMHO, being hit by a car is the unit measure of risk - if something is a couple of digits less dangerous than cars, it's livable with.
    – Vorac
    Commented Jan 9, 2014 at 10:22
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    Yeah, this is my point. I've always thought of water purification for something to do in stagnant or slow moving water of questionable quality. I'd never think to do it in a fast running mountain stream in a National Park. Reading that article made me wonder am I in the minority? What are the stats on this.
    – user2766
    Commented Jan 9, 2014 at 10:26
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    @Liam I think mountain water is pretty safe. In California, we are careful that there aren't any cattle ranches up stream. But other than that, mountain water is pretty safe for backpackers. (My wife is a doctor and wants me to remind you how easy it is to purify your water anyway.) Commented Jan 9, 2014 at 16:21
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    I would err on the side of caution with this one. Like everyone has been saying, it is pretty low risk due to the things you mentioned. However, @AM_Hawk is right. I've come across a few streams that look completely safe and then found an animal carcass snagged a bit further upstream. Still not worth the risk in my books! Commented Jan 3, 2015 at 7:34
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    +1 @verrucktfuchs - that exact scenario happened when I was walking some Munro's in Scotland. Drinking, drinking, drinking...100 metres higher round a bend; dead sheep in the water. Bleurgh Commented Jan 4, 2015 at 13:12

6 Answers 6


I think your assumptions are correct. To my knowledge in a mountain environment you are quite safe as long as you follow some simple rules, which you mostly already named:

  1. The water was not standing, i.e.

    • it comes from a stream that is rather fast and
    • the stream is big enough that it is not just a connection of puddles or ponds where the water rinses from one to the other.
  2. There are no sources of contamination upstream. As you mention, thinkable sources are

    • agricultural areas where fertilizers or herbicides could have been used
    • meadows, where cattle was present
    • mining
    • (quite unlikely) the water runs through some geologic formation where harmful stuff gets washed out.

As long as you have checked such stuff, you should be quite safe with flowing water in mountain areas. Even the streams of melting water that build up on the surface of glaciers on summer afternoons will be rather fine then.

  • thanks, well that validates my assumptions. I think I'll leave off marking this as the answer, just to gauge a bit of opinion.
    – user2766
    Commented Jan 9, 2014 at 10:39
  • One other source of contamination worth watching out for in mountainous regions: on a few occasions I've seen dead sheep that have obviously fallen down a mountainside; you don't want to be drinking water downstream of that. I've drunk water from mountain streams on numerous occasions, but in most cases I'll stick to my own water on the way up, and drink from the streams on the way down when I've seen that it's clear further up. That's not to say I don't have enough with me for the whole trip, but water from streams is colder and may taste fresher than what's been in my flask all day. Commented Mar 30, 2016 at 14:25
  • I'd be cautious about glacial meltwater. It tends to contain substantial amounts of powdered stone, which acts as a laxative.
    – Mark
    Commented Feb 11, 2017 at 3:18
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    @Mark Well, I think it depends. If you mean drinking from the torrent or river that runs out of the glacier somewhere at its edges, I absolutely agree that there's lots of powdered stone in there. But that shouldn't hold for small rivers on the surface of the glacier itself, since the stone powder should only exists near the sole of the glacier. Commented Mar 8, 2017 at 19:45

The bottom line is there is always SOME risk. Whether to take that risk or not is your choice.

Fast running + isolated + high elevation = prettttty low risk.

With that said the biggest concern is, unless you are drinking right from the source, you have no idea what has happened upstream from you. There could be a dead animal snagged in the stream, animal and human feces etc...

I have drank from mountain streams and not gotten sick, but I have seen others partake and get real sick...The choice is yours.

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    I do think the animal feces thing is always a bit overplayed. If there is 100,000 gallons going down this stream in x amount of time, it's going to take a hell of a lot of animal faces to pollute a stream. Dead animal maybe, but again the shear amount of water means (to my mind anyway) it'll have to be a pretty big dead animal pretty close to where your drinking to introduce enough bacteria to make you sick.
    – user2766
    Commented Jan 9, 2014 at 17:18
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    @Liam I do agree with you, I just wanted to point out that there still is a risk, how big of a risk is up to the individual to assess and use good judgement to decide how to proceed.
    – AM_Hawk
    Commented Jan 9, 2014 at 17:25
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    @Liam. The thing is the bacteria will multiply inside your body. All it takes is for you to ingest a small amount, and as long as that small amount survives inside you long enough to start replicating, then you will have a problem. How do you think people pass colds and flus to eachother? All it takes is a sneeze, giving you just enough germs to get going ... Commented Jan 2, 2015 at 22:57

I have traveled the back country for 4-6 weeks a year for 30 years, mostly in the Canadian rockies and on the pre-cambrian shield. I've never used water purification, nor did we generally in our group. We've had some cases of the runs over the years, but the spread out nature made it unlikely it was a water source that did it. Far more likely bad hygiene on part of the kids.

Once on the Nechako we got started in the morning, continued to track upstream. Around the corner, not 100 yards upstream from our camp was a bloated cow on a gravel bar. Bossy was well past her Best Before date, and was likely winter kill left on the gravel by falling spring runoff.

Despite having had diluted cow for the previous evening and breakfast, no one got sick.

There are parts of the world where you have to be concerned about water. Not here.

A large part of the risk comes from substantial exposure. E.g. If you get 6 bacteria, your immune system stomps all over them. If you get a dose of 6 million bacteria (About half a serving of active culture yogurt) AND if they are a variety that is toxic, you are in trouble.


No you don't.

We live in a small Swiss village of 380 inhabitants

All the water we use comes from above the village as spring water

This is piped to our house and to a village water trough (for animals and humans )

Tourists often ask to drink from our house because the idea of drinking from an outside water source is euuk (yes many are American)

However a) it's the same water (origin)


b) it runs continually from a clean spigot into the trough so it is cooler, it is fresher (it has not been standing in a metal or plastic pipe so fewer contaminants), it is more highly oxygenated. it is free - we pay for our metered use.


The one shortcoming I find reading through many of the other answers is that they seem to skip over a major source of pollutants in backcountry streams: human waste. While you might not be downstream of a cattle feedlot, there's a chance that a human (or other animal) took a dump just upstream from you. Doubly so if you've been hiking along an established trail.

For example, here's a USGS study. While not particularly far into the backcountry, both of these creeks would objectively appear to be pristine, swift moving mountain streams. Pay particular attention to the bit about "fecal coliforms" and keep in mind that there are more people in the backcountry than there were 20 years ago...

Abstract: This study was conducted in Grand Teton National Park during the summers of 1996 and 1997 to investigate the water quality in two high human use areas: Garnet Canyon and lower Cascade Canyon. To evaluate the water quality in these creeks, fecal coliform, Giardia lamblia, coccidia, and microparticulates were measured in water samples. No evidence of fecal coliform, Giardia lamblia, or coccidia, was found in Garnet Creek. The water quality and general water chemistry of Garnet Creek was similar to the reference site. No Giardia lamblia or coccidia were found in Cascade Creek, but fecal coliforms were present. The isolated colonies of Escherichia coli from Cascade Creek matched the ribosome patterns of avian, deer, canine, elk, rodent, and human coliforms.

Personally, I'll deal with purifying my water (and bringing hand sanitizer!) in order to not deal with diarrhea while backpacking.


For certain contaminants such as Giardia, it doesn't matter whether the water is running fast or slow. You will get infected regardless. So this idea about "fast water" is a myth that is not correct.

If you are drinking close to the source of streams and there is not any mining activity or industrial pollution nearby, then OF COURSE you are drinking clean water. Assuming horses and cows aren't defecating in it further upstream from you. The appearance of things in the water that you need to worry about is usually associated with activity from animals pooping in it or humans putting things in there that they shouldn't. Particularly when talking about cold mountain streams near their source.

My wife is from a small town in the coffee-growing region of Colombia, and her family has always, and continues to, get their water by taking a bucket up to where the small springs shoot out of the sides of the mountains (the places where the springs are born). They don't boil or filter this water, they drink it straight.

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    -1: The fast water idea is not a myth. It is correct for many, many contaminants, so slow or standing water will have a bigger probability to get you infected. Since we are talking about probabilities and risks anyways, I think its a good idea to stick to. Commented Apr 18, 2015 at 12:23
  • @PaulPaulsen I would agree with you on that. It would make sense the probabilities are higher with standing water. Commented Aug 10, 2017 at 18:14
  • And I, with more than 2 years passed since then, would now agree that it doesn't help much if the water gets running fast again if it has been standing for a longer period and getting contaminated further upstream. Maybe you would like to edit your answer a little bit to reflect this, then I would be happy to retract the downvote :) Commented Aug 11, 2017 at 22:04