The Earth is attracted to the Moon, and vice versa. The Earth-Moon attraction depends on the distance that separates them. Thus, the closer an ocean is to the moon, the more it will be attracted to it. At the moment of a strong attraction, the earth's crust is also raised on average by 40cm. So the water level goes up, it's high tide. The Earth continues to turn and is no longer in front of the Moon. The attraction diminishes, the waterfalls: it is low tide. Photo credit: Ben Thouard / The Expl