Answer:
fertile soil flooding
Step-by-step explanation:
Most ancient civilizations developed alongside rivers. The main reason why is that these rivers periodically flooded the sorrounding areas, making the soil more fertile. This physical phenomenon allowed the cultivation of more and better crops, which increased the wealth and complexity of the civilization.
Examples are: Ancient Egypt and the Nile, Ancient Mesopotamia and the Tigris and the Euphrates, and Ancient China and the Yellow River.