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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... – Bananenaffe 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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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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