The correct answer to this open question is the following.
Unfortunately, you forgot to mention what civilizations you have studied. We do not know that information. Just you.
However, trying to help you we can comment on the following.
In general terms, the development of city-states is different from river valley civilizations in that some were more powerful than the others in the way they grew, prosper, and developed, and in the social and political structure and organization.
We have teh case of the oldest civilization on earth, the Sumerians.
The Sumerians settled in the middle of the Tigris and Euphrates Rivers, in the Middle East region, modern-day Iraq. They settled there because of the many benefits of agriculture.
After the flooding of the rivers, the soil was very fertile to grow crops. That allowed them to establish powerful city-states as was the case of Lagash, Eridu, Nippur, Ur, Kish, and Uruk.
These city-states built impressive buildings called Ziggurats as symbols of religion and power in the many city-states. Some Ziggurats were symbols to honor their gods.
