While rivers provided life, they also brought death. Settlement on the plains, which enabled people to take advantage of the rich alluvial soils, also exposed crops and villages to the risk of catastrophic floods. Gilgamesh, the earliest surviving epic tale, tells of a great flood unleashed by God to scourge the sinful in Mesopotamia. Myths and legends of huge floods are common to many cultures around the world, from the Old Testament Jews to the pagan Norse and the indigenous people of the Americas. The damming of the world has brought a profound change to watersheds.
Nothing alters a river as totally as a dam.
A reservoir is the antithesis of a river - the essence of a river is that it flows, the essence of a reservoir that it is still. A wild river is dynamic, forever changing - eroding its bed, depositing silt, seeking a new course, bursting its banks, drying up. A dam is monumentally static, it tries to bring a river under control, to regulate its seasonal pattern of floods and low flows. A dam traps sediments and nutrients, alters the river's temperature and chemistry, and upsets the geological processes of erosion and deposition through which the river sculpts the surrounding land.