Why are the Tigris and Euphrates Rivers dangerous to the Sumerians

The Sumerian’s City-States depended on the swamp lands located between the Tigris and Euphrates rivers’ Fertile Crescent. Due to the constant flooding, this Fertile Crescent formed by making the Persian Gulf to recede from its previous shores. The floods brought much fertile top soil down from the mountains that gradually filled that section of the Persian Gulf. The Sumerians had built high walls around their city to divert the periods of flooding waters. This swamp land being plowed and used for Sumer’s agricultural land, that fed their people, had gradually brought up a salty water that would kill their crops. The land became not fit to grow anything. So the Sumerians had to relocate to a better land for to grow their crops as to feed their people.