These days you can just bisect a segment of land horizontally and vertically and boom, build a city. But 5,000 years ago, civilizations were almost always associated with rivers, whether that's the Tigris and the Euphrates, the Yellow River, the Nile, the Amazon Basin, the Coatzacoalcos. Gah, I was doing so good until I got to Coatzacoalcos! Coatzacoalcos. Coatzacoalcos. Maybe. Why river valleys?