Khwarezm was highly dependent on the Amu Darya river regime. Since the period of the highest level of the river coincided with that of the vegetation, the knowledge of astronomy was crucial for successful agriculture on the land. The authors of this study assume that in the ancient name of Amu Darya May flood (“The Flood of the Star”) the Star, according to the old Iranian tradition, meant the Pleiades, not Fomalhaut (α Piscis Austrini) as it has always been considered. May flood was called “The Flood of the Star” in the second half of the I millennium BC, when it coincided with the heliacal rising of the Pleiades. The advantage of the Pleiades as a marker consists in the fact that this cluster cannot be confused with any other object of the night sky. The asterism of the Pleiades was well identified even in the ancient cultures with rudimentary knowledge of astronomy. The Pleiades could have been used as a marker of Amu Darya May flood for some hundreds of years. Later because of precession the Pleiades eventually began to rise after the flood, but the latter kept its historical name.