<P> A river valley civilization or river culture is an agricultural nation or civilization situated beside and drawing sustenance from a river . A "civilization" means a society with large permanent settlements featuring urban development, social stratification, specialization of labour, centralized organization, and written or other formal means of communication . A river gives the inhabitants a reliable source of water for drinking and agriculture . Additional benefits include fishing, fertile soil due to annual flooding, and ease of transportation . The first great civilizations, such as those in Mesopotamia and Egypt, all grew up in river valleys . </P> <P> Civilization first began in 3500 BC, which along the Tigris and Euphrates rivers in the Middle East; the name given to that civilization, Mesopotamia, means "land between the rivers". The Nile valley in Egypt had been home to agricultural settlements as early as 5500 BC, but the growth of Egypt as a civilization began around 3100 BC . A third civilization grew up along the Indus River around 2600 BC, in parts of what are now India and Pakistan . The fourth great river civilization emerged around 1700 BC along the Yellow River in China, also known as the Huang - He River Civilization . </P>

Which of the following was most significant in allowing river valley civilizations to flourish