The low river issue 2 or 3 summers ago was overblown. When there is not enough water, the production can be slowed down to use less water.
Regarding what happened during that heatwave, there are, rightfully so, regulations on how hot the water can be to be dumped into regular rivers. These regulations ensure the protection of the marine life and ecosystem of these rivers. During the that summer, the rivers' natural temperature was almost as hot as the threshold set by these regulations, so they could not dump any hot water into the rivers. But there are add-ons solutions to this in the future, such as cooling ponds where they would let water temperature dissipate before dumping it into the rivers.