Sorry for the double post, but I had another idea.
How about this: everyone's in the valley, and then a few more watering places show up. nobody thinks nothing of it, until more and more show up. but when the waters get to the point where nests are swept away and families are forced to move to higher ground, everyone knows something is wrong. food becomes hard to find, they constantly have to travel to ensure they won't be swept away in their sleep (not everyone as a group, but any family/herd who feels the need), and then the problem isn't resolved until a few of the kids (and some grownups looking for the kids) go into the Mysterious Beyond.
Idk, I know it sounds A LOT like number 3, but with a different rising action and spark-off. Thoughts?