Hey there! Great question!
Sumer met the criteria of a cilvilization because of their cities, like Ur, Uruk and Kish, they're surplus of food, due to their irrigation system and their division of labor, otherwise known as jobs.
The Tigris and Euphrates river both placed a huge part in how the development of Mesopotamian civilization begun.The rivers were able to basically stretch itself towards the farmlands. The farmers could use the water and it led to the construction of canals, aqueducts etc. That then eventually led to a cilvilization.
Hope this helped! Put this in your own words!