Rivers can and do dry up or reduce significantly in flow.

I ment the first sovjet reactor, were they literally diverted a river through the reactor core. Polluting all villages and a sea down stream forever.

Oh that's really ugly, do you know which reactor this was?

They are referring to Mayak, and however ugly you're picturing, it's probably worse: a Chernobyl-level radiological release, but on deliberate as operational procedure.

https://en.wikipedia.org/wiki/Mayak

Holy crap, I never knew about this. Wow. That's a level of irresponsibility that I did not think would exist even in that part of the world. Incredible.

I think the first US plutonium producing reactors at Hanford also used direct cooling by river water.