Answer: Summer rainy season
Explanation:
When rain came in the summer it led to floods in both the Nile and the Tigris and Euphrates. These floods brought fertile silt for growing crops and was the reason the ancient civilizations of Egypt and Mesopotamia were able to take hold.
Most of these areas that rely on floods do not have much in the way of rain in their region for instance Egypt. So when the summer rains come and flood the rivers further upstream, they benefit from it as the planting season will have fertile soils.
