I always wonder if myths about "rivers turning into blood" is just an attempt to explain, or even just a metaphor for, a bloom of red algae.
In Germany, we sometimes have HABs in summer, in some of the cities like Hamburg. People are then told to not go out and swim or even be careful with drinking water right from the tab. Of course, not everyone listens.
Although in this case, it's usually not red but blue green algae.