Instructions
Drag one or more adult villagers to the debris blocking the stream, located just to the left of the flutes on the cliff.
Drop the villagers on the debris to instruct them to clear it. Remove all of the debris.
Take a villager with trainee or better skills in both science and construction to the large rock, called a keystone, located where the stream had previously ended on the northeast side of the island. She picks up the stone and puts it in the gap to the west of the tree, filling the lagoon and restoring the stream's proper path.
Leave at least one villager at the location of the former obstruction in the stream to keep it free from debris.