The first river civilization arose in Mesopotamia sometime around 4500 BC. This civilization is known to us as Sumer. Sumer’s economy was based on irrigated agriculture using water from the Tigris and Euphrates rivers. The creation of this civilization involved the creation of a more complex social order.
When the people of Sumer learned how to use the rich soil of the river valley for agriculture, they increased their ability to produce food. When they did this, they also increased their capacity to support large populations of people. When they came to have a large population, they also came to need a more complex society.
In society with a large population, there needs to be a central government. Someone has to keep order is society and protect the society from outside attack. This means that society becomes stratified with masses of farmers supporting a relatively small governing elite. The farmers also typically have to support religious elites. The surplus of food produced by the farmers can also support people who specialize in things like making pottery or working with metals. In these ways, intensive agriculture leads to larger populations and a more socially and economically complex society.