The main factor that led to the development of the four major river valley civilizations was its closeness to rivers, which was a food source, its flooding allowed for fertile soil, allowed easy transportation and cities near them were easier to defend.
The fact that fewer people had to dedicate to the production of food allowed some of them to develop other activities, such as the construction of building and cities, metal working, trade and social organization. This led to a social division, the ones that worked and the ones that ruled, creating intricate systems of governments and religions that ideologically sustained those in power.
