Plentiful water and the enrichment of the soil due to annual floods made it possible to grow excess crops beyond what was needed to sustain an agricultural village.
This allowed some of the people to engage in non-agricultural activities, such as the construction of buildings and cities (the root of the word "civilization"), metalworking, trade, and social organization.
[1][2] Boats on the river provided an easy and efficient way to transport people and goods, allowing the development of trade and facilitating central control of outlying areas.
Eventually, they constructed irrigation systems to exploit the two rivers, transforming their dry land into an agriculturally productive area, allowing population growth throughout the cities and states within Mesopotamia.
A major impetus for the tribes to unite into a single kingdom by around 1700 BCE (Erlitou culture, a Yellow River civilization) was the desire to find a solution to the frequent deadly floods.