In addition to the other concerns of evaporative cooling, there is one issue with a simple water cooled loop that just runs river water through a heat exchanger and then dumps it again. You can only raise a river's temperature so much before it becomes a problem for the ecosystem. This issue has in the past forced power plant shutdowns, so it's not that simple.

https://www.ans.org/news/article-6268/french-nuclear-plant-l...