From Bulbapedia, the community-driven Pokémon encyclopedia.
Blue Moon Falls (Japanese: 青い満月の滝 Blue Fullmoon Falls) is a Johto location exclusive to the anime.
Its sole anime appearance was in Once in a Blue Moon. Ash was upset that the GS Ball, which had been entrusted to him, had been taken by a Quagsire. Officer Jenny explained that all the Pokémon was doing was taking it to Blue Moon Falls for an annual celebration. Ash, both intrigued and eager to get the Ball back, decided to investigate this further.
Sure enough, he and his friends stumbled across Blue Moon Falls. They witnessed hundreds of Quagsire with round-shaped objects. At midnight, they tossed the objects into the air and used Water Gun to send them over the falls. The following morning, all the objects, including the GS Ball, floated safely back down stream.
Pokémon seen at Blue Moon Falls