Three thousand waving the rocks beneath the Mekong river. Pho Sai district. Ubon Ratchathani. Caused by the vortex erosion. A pool of more than 3,.000 wells and is visible during the dry season the water dry up. Such rocks are like mountain water broke through the middle of the Mekong River. The locals call it. Grand Canyon river water. Which this phenomenon occurs from about October - May.