The day following mystery time, villagers will start talking about trash and the absence of fish in the river.
After fishing up some trash from the river, you may spot a quite enraged green monster which usually moves around the west side of town near the river. It cannot speak recognizable words but rather a jumbled mess of angry syllables.
Keep fishing up trash. Gradually, the mokele-mbembe's mood will lighten in direct correlation to the amount of trash fished out of the river. Once the fish start to reappear and/or the monster, upon being spoken to, is delighted with the cleanliness of the river, go to the waterfall to end the mystery.
You will see the mokele-mbembe looking at the water as it begins to sparkle. It then makes its transformation to the Goddess of the Waterfall. She thanks you for cleaning the river and gives you her blessing before leaving. Mystery solved!