Coatzacoalcos. Coatzacoalcos. Maybe. Why river valleys? They're flat, they're well-watered, and when they flood, they deposit nutrient-rich silt. We'll have more to say about most of these civilizations later, but let's talk about this guy, the Indus Valley Civilization, because it's my all-time favorite. The Indus Valley Civilization was located in the floodplain of the Indus and Suwarti Rivers, and it was about the best place in the world to have an ancient civilization, because the rivers flooded very reliably twice a year, which meant that it had the most available calories per acre of pretty much anywhere on the planet.