How did the geography of the Fertile Cresent help the Sumerians to prosper?

The Euphrates and Tigris Rivers were the source of life to the Sumerians, giving them food (fish) and water. Rich soil near the banks of these rivers provided land for crops. I know that this is a question in the textbook World Studies; The Ancient World, so please re-phrase this answer and make it your own instead of cheating.