The Tigris and Euphrates rivers reach their lowest levels in September and October and normally flood in March, April and May. The rivers can carry up to 40 times more water when they are high than when they are low. In addition, the flood in one year can be up to 10 times larger than the flood of other years. If the Tigris and Euphrates flow unchecked, they can flood with such force that entire towns can be inundated with water. In ancient Mesopotamia, irrigation dikes and other structures helped people better control the rivers and create fertile farm land, which encouraged the first cities to form.