This was done to prevent badly affecting the ecosystem of the rivers. The water from the river is used to cool down the plant, but the resulting warm water is poured back into the river. The temperature of the water going back to the river is carefully monitored and during heat waves it may be decided to shut down the plant. By the way this is not specific to nuclear at all, coal plants work the same, but France is the major exception in the world with 74% electricity coming from nuclear, and most of the rest coming from dams (extremely low carbon footprint for electricity). Some plant designs only use closed-circuit cooling which doesn't have this problem. The towers people usually associate with nuclear plants, is in fact a cooling tower that uses the air to cool the water. Again despite popular belief, these cooling towers are not just for nuclear, they are also used for coal plants (any thermal power plant)