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.