The flooding patterns of the Tigris and Euphrates rivers are unpredictable, but the Nile River's flood cycle is predictable. Floods on the Tigris and Euphrates can also vary from violent to minor, involving different amounts of water compared with the predictable amount of water in the Nile's floods.
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.
Unlike the unpredictable flooding patterns of the Tigris and Euphrates rivers, the Nile overflowed about the same amount of water once a year in ancient Egypt. People who lived along the Nile constructed irrigation projects so they could use the river effectively. These systems also helped the Egyptians build large cities and expand their civilization.