All of the great early civilizations were born in alluvial valleys — notably the Tigris — Euphrates delta, then the valleys of the Nile, the Indus, and the Yellow River (Figure 1.2). The soil in those valleys is fertile, but must be irrigated and drained, and this requires the establishment of coordinated water management. But there is another consideration: populations near the river are exposed to the deadly threat of flooding. Flood protection is therefore another fundamental need, demanding societal organization of significant manpower, and attracting population agglomeration to sites that can readily be protected. All of this implies a certain collective and hierarchical organization.
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