Between 4000 and 2000 BC, the first urban civilizations arose independently along the Tigris, the Nile, the Indus and the Yellow River. All of these great rivers are prone to seasonal variations in flow, with flood alternating with drought. To maximize agricultural production, it was necessary to build dams to store the flood waters in reservoirs to water the fields during the dry season. Once this technology was developed, farmers found that they could move into drier areas, such as southern Mesopotamia.