<P> Civilizations tended to grow up in river valleys for a number of reasons . The most obvious is access to a usually reliable source of water for agriculture and human needs . 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 for some members of the community to engage in non-agricultural activities such as construction of buildings and cities (the root of the word "civilization"), metal working, trade, and social organization . Boats on the river provided an easy and efficient way to transport people and goods, allowing for the development of trade and facilitating central control of outlying areas . </P> <P> Mesopotamia was the earliest river valley civilization, starting to form around 3500 BC . The civilization was created after regular trading started relationships between multiple cities and states around the Tigris and Euphrates Rivers . Mesopotamian cities became self - run civil governments . One of the cities within this civilization, Uruk, was the first literate society in history . Eventually, they all joined together to irrigate the two rivers in order to make their dry land fertile for agricultural growth . The increase in successful farming in this civilization allowed population growth throughout the cities and states within Mesopotamia . </P> <P> Egypt also created irrigation systems from its local river, the Nile River, more intricate than previous systems . The Egyptians would rotate legumes with cereal which would stop salt buildup from the fresh water and enhance the fertility of their fields . The Nile River also allowed easier travel among the civilization, eventually resulting in the creation of two kingdoms in the north and south areas of the river until both were unified into one society by 3000 BC . </P> <P> Much of the history of the Indus valley civilization is unknown . Discovered in the 1920s, Harappan society remains a mystery because the Harappan system of writing has not yet been deciphered . It was larger than either Egypt or Mesopotamia . Historians have found no evidence of violence or a ruling class; there are no distinctive burial sites and there is not a lot of evidence to suggest a formal military . However, historians believe that the lack of knowledge about the ruling class and the military is mainly due to the inability to read Harappan writing . </P>

Why did the nile river become a center for early civilization
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