Thousands of families have ____ the valley since the river burst its banks.

Բացատրություն

Escaping from danger is flee — fled — fled, which uses one past form for both slots, so the participle after have is fled. Flown is the participle of fly — flew — flown and is about moving through the air, while flowed comes from the regular verb flow, used of water moving steadily rather than of people leaving.