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I was hiking in Switzerland yesterday and came across a small river which holds very little water on most days (or even none) and grows a lot after rainfall. The day before yesterday it rained for the first time in weeks, so the river held a lot of clear non-smelly water. I was wondering if it was safe to drink the water after a long time without rain.

The reason for this: I once lived in a house which got its water from a natural spring. Every so often we had to boil the water because after long periods of drought the first rain washed a lot of chemicals from a nearby road into the spring. The water then smelled and looked fine to drink but contained small particles of rubber and oils from cars.

There is no road near the river in question, but maybe the forest could accumulate similar contaminants (human waste, dead animals, etc.) and wash them downstream with the first rain after a drought.

This question is about drinking water in the alps, but does not address the issue with contaminants that might not be visible/smellable in the water.

This paper mentions on multiple occasions the problems of contaminated water after rainfall in urban areas. Maybe there is an equivalent for situations in the wilderness.

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    The way to be sure is to have it tested. – paparazzo Jul 4 '18 at 14:41
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    This is quite hard to answer with the given information. You may want to ask about the specific river in general "Is it safe to drink from the river XYZ near ABC? And if so, do I have to be careful after a drought?" – OddDeer Jul 5 '18 at 5:35
  • @OddDeer I have added a paper which specifically addresses the issue of contaminated water in urban areas after rainfall. I would love to see something like this for non-urban areas... – Wolter Jul 5 '18 at 7:35
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    I don't think boiling had much of a relevant effect against oil or rubber particles. If your worry is now about something like fecal matter, that process becomes more useful. – helm Jul 5 '18 at 8:16
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    No source, or I would make this an answer: I don't think the issue (countryside) is so much long draughts as recent heavy rain. Water from heavy rain will run above ground, flushing organic (or non-organic) matter into the streams. Flow from gentler rains will mainly reach streams through the ground, being filtered in the process. – Guran Jul 5 '18 at 8:32
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The reality is that if there is a trail, there will be human waste improperly deposited near that trail. Recent heavy rain will wash this waste into the stream. Also, stream beds are where gravity naturally deposits anything that can be moved by gravity. Stream beds are nature's garbage collectors.

All the factors that make urban streams risky after drought apply in the wild. There may be much less of a risk compared to urban streams, but rain after drought is likely to be a high risk time to drink from a stream without filtering.

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  • Why would human waste be more hazardous than wolf, bear, moose, wolverine, beaver, squirrel, marmot, crow, eagle, chickadee...? – Sherwood Botsford Jul 13 at 21:47
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Before drinking from a stream, check your map to see where that stream comes from.

Does it flow through a settlement or even worse past something industrial? Does it come from a lake? Could be polluted with anything, including things you might not be able to get rid of by filtering or cooking. If you have a choice, avoid! (if this is a survival situation: Most of the consequences you could feasibly expect from drinking contaminated water should be survivable if you can expect to get proper medical care in the next couple days. So if you are just about to die from dehydration, then the only water which could possibly make your situation worse would be salt-water)

Does it flow past meadows? If those are used for farm animals, then the water might be contaminated with E. coli bacteria from animal feces. Boiling the water for 30 minutes will kill those. There are also tablets you can buy which promise to kill bacteria in drinking water.

Is the spring close and does the stream go mostly through forest or other forms of terrain not used by humans? Probably safe to drink. There is of course always a remaining risk of pollution through wild animals or inconsiderate humans, which increases proportional with the length of the stream. Use your own judgment.

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I have spent somewhere around 1000-1200 nights in the Canadian Rockies, Prairies, boreal forests and tiaga. I rarely have used either a water filter or sterilizer tablets. Nor have people on our trips.

Have we had stomach troubles? Some. Nausea, gas, frequent and runny stools. Usually stopped within 2 days of telling people to clean their cups after eating. (As backup for this, staff, who were less likely to lend or borrow a student's cub were far less likely to get ill.)

On the Nechako we were tracking upstream in farm country. Stopped for lunch. Drank the river. Around the next corner found a dead cow on a gravel bar.

Once group on the Chislata found so many dead salmon in river that the water tasted fishy.

So, are you safe? Probably not. But I would spend more time worrying about sudden snow storms, breaking your ankle, or being attacked by a mountain goat in rut.

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  • Found a dead horse under the water (so pretty fresh) in a pristine small lake in Desolation Valley wilderness area in the Sierra. Don't recall whether we had drunk from the lake before getting to where we discovered the horse. Anyway, I concur with your experience. Of course, there are more human visitors everywhere these days... – Drew Jul 21 at 5:51

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