The people of Sumer are among the earliest denizens of Mesopotamia. By about 4000 BCE, the Sumerians had organized themselves into several city-states that were spread throughout the southern part of the region. These city-states were independent of one another and were fully self-reliant centers, each surrounding a temple that was dedicated to god or goddess specific to that city-state. Each city-state was governed by a Priest King.
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Rivers supplied not only water for the colonist, but transportation which also led to trade. Also a lot of wild life are around river which gave them food also weather it be fish in the rivers or animals that come to the river to drink. So rivers provided water, transportation, and food.