The Stream is the second puzzle in Virtual Villagers 4.
Head to the cascading waterfall.
Pay close attention to where it ends.
Look for the stream obstruction, left of the wind flutes, and drag a villager on it so they can begin clearing it.
You can use more than one villager to speed up the process.
Once the obstruction has been cleared, the water will surround the tree.
Head to where the waterfall used to end and you will find the keystone.
Drag a villager who is both an adept builder and an adept scientist to the stone and they will plug the hole to the left of the tree thus restoring the normal flow of the stream.