The answer is:
Industrial Waste,
Usually come from the residual of manufacturing companies that irresponsibly dumped into water supply (lake or rivers)
Human sewage
At that time, many people drop their feces into the river rather than usign actual toilets. This contaminated the water on both those rivers.
agricultural runoff.
The chemical that is used by farmers near that rivers is absorbed by land and slowly brought to the rivers. This increase the toxicity level of the rivers and promote unnatural growth to the algae that live in it.