The river is not warm or warmer than the air. Heat pumps are amazing at extracting thermal energy. I think water is very dense compared to air, thus making the processes more efficient in such a large scale.
Ground source heat pumps are limited because the ground they have chilled stays stubbornly in the same place, so the only way you can extract more heat from it is to make it even colder, which gets less efficient. Watercourses don;t have that problem.
Not always good for the local ecosystem without mitigation, but at least one Japanese reactor allowed local colonisation by tropical fish and local legend said the same about Sizewell.
Sizewell C claims to plan recover waste heat and use it for carbon capture somehow, about which all I can say is a big old hmmmmm.
Not quite the same thing, but there is a tropical greenhouse in the south of France that used to be heated by cooling water from a nearby uranium enrichment facility: https://fr.wikipedia.org/wiki/La_ferme_aux_crocodiles (unfortunately not available in English).