HM the thing that came to my mind was acid rain. If the PH is off just a little in the rain it could do this. Acid rain isn’t what everyone thinks it is. It doesn’t burn threw skin or all that. It is just an acidic PH level. I highly doubt it is that but that is what popped into my mind.
I doubt it was acid rain. This event seems to be confined to a two mile long section of the stream. That’s plenty bad enough, however if it was the rain I think it would have affected a much larger area.