Depends on what happened in that video. I would rewatch it and write about what the video said about flooding and relate it to the river. Could be an ice jam or something else slowing the flow of water suddenly opened up. A breaking dam? Flash floods from an abundance of rain. As the amount of water in the river increases, so does the flow rate. If the amount of water suddenly increases more than the river was used to/it could hold, it will flood.
Rewatch the video and see if this relates at all :)
