[boxed]

I mean, "hot water" is a bit vague. What kind of temperature differentials are a problem? 0.01 degrees C 50 meters down stream I'm guessing isn't. 100 degrees C I'm assuming is super bad. What is the effect size? How much do we have to dump into a river must depend on the flow rate of the river, the dumping, the temperature of the river and the temperature of the dumped water, etc, etc.

And nothing at all is too stupid of a question to want an answer to. That way leads to people screaming at each other over social media instead of engaging with facts.