This happens in these states every so often. Fertilizer or pig shit or whatever wipes a river clean of all life.
A personal experience: I stopped washing my hair as reguarly as I used to when I was in Iowa, since it had started falling out. I thought, ageing, and what not. But when I stopped washing my hair more than twice a week, the hair-fall stopped. This was more than 20 years ago, and I still haven't balded much after that. I have always wondered whether it was related to the water quality.
It’s preventable.