The plants and the fish can die. On the short term, the oxygene content of the water is the problem. The oxygen content decreases with increasing water temperature. So beyond a certain temperature, the oxygene levels can go below the levels required for survival, the animals and plants suffocate. Especially in lakes it can happen, that in these weather conditions basically all fishes can die at the same time. Agitating the water helps to maintain higher oxygene levels.

> What happens to plant and animal life if we raise the river temperature?

The entire ecosystem changes, possibly hosting invasives / foreign species and killing the native ones (or becoming inhabitable).

That is why — in france at least but probably not solely — there are pretty strict limits on the allowable difference between input and output water temperatures.