Heating water into steam takes energy, a lot of it. Like how a pot of water takes a while on the stove to boil, even though the stove top is putting out a lot of heat.
Trying to evaporate a running river would take an incredible amount of heat, and to make that heat would take and incredible amount of energy. It would be like trying to keep a pot of water boiling while constantly adding cold water to it.
So, while you technically could evaporate water out of a river to bring it down from flood stage, it would take too much energy.
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