Answer:
This is a very unlikely situation, since the plant must be really large and the river probably didn't carry a lot of water in the first place. But even if this was possible, it would be illegal for a company to use 100% of the natural resources available. No law or regulation (municipal, state or federal) would allow such thing to happen and assuming it got to court, the court would rule against the company.
Since you need an environmental impact report before you start building a factory, then it would be unlikely that the factory or plant was legally authorized to operate in the first place. The only option is that they built a dam and that is highly regulated.
