The earliest civilizations grew up along great rivers, and the reasons are not hard to see. The Nile, the Tigris and Euphrates, the Indus, and the Yellow River all provided reliable water for drinking and irrigation in otherwise dry landscapes. Annual floods spread fresh silt across the fields, renewing the soil and making it possible to harvest surplus grain year after year. That surplus fed craftsmen, priests, and officials who did not need to farm, and cities formed around temples and granaries. Rivers also served as highways, carrying stone, timber, and trade goods far more easily than any road could.
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