How did the environment influence the rise of city states in Southern Mesopotamia

Mesopotamia was dry except for the two rivers, so the Sumerians needed to stay in a place where they could get water easily.The Tigris and Euphrates river were higher than the plains,and so people realized that they could get water more easily in the plains using irrigation system.Also, civilization emerged in Mesopotamia because of the surplus. Because Mesopotamia had such rich farmland, it was able to produce more food than people could actually eat. This meant that fewer people were needed to produce food to support the population. So more people could do other jobs: from heavy labor building things to skilled craftsmen making tools, furnishings, clothing, and other stuff. Not everyone needed to work, they can trade food with others.