I'd assumed that the animals died in the valley before it flooded, and they came back where they'd originally died which is now under water - hence why they'd all drowned.
I'm probably completely wrong...
I have a number of French colleagues working over here at the moment and it's the highlight of their week, as well. They also find it utterly hilario that the ad breaks reciprocate with French subtitles for their benefit. Trouble is, as an aide to their language learning regime it does my head...