Subtitles section Play video Print subtitles The world is divided into two kinds of people: those with innie belly buttons, and those with outies. Rivers also have innies and outies – not belly buttons, but mouths: where rivers flow into the sea, the land either pokes out or bends inward. But rivers don’t have umbilical cords, so why do they have innies and outies? Well, coasts are the front lines between two opposing forces – land and water. In order for the ocean to invade the land, sea level either has to come up, or the land has to sink down or be eroded1 away. And in order for the land to advance into the ocean, sea level either has to drop, or the land has to build (or be lifted) up. Obviously, if sea level drops and then rises back again, there’s no net gain on either side – but things get more complicated when a river joins the battle. For example, during the last ice age, sea levels fell by over 120 meters, and rivers cut deeper and deeper valleys to reach the falling seas. Then, about 18,000 years ago, warming temperatures began to melt the ice, and the rising seas flooded river valleys around the world, creating giant estuaries and giving us the innie-riddled coastlines we have today. But when the steady landward march of the seas finally began to slow, about 7,000 years ago, the coastlines around the mouths of some rivers began to gain back some ground. The key factor was the sediment the river dropped as its current slowed at the entrance to the sea. Where the sediment supply was big enough and the ocean was calm enough, the dropped dirt piled up, eventually forming new land that both lengthened the river and divided it in two. Dirt continued to drop out and build up at the mouths of both channels, splitting the river again...and again...and again, creating a new lobe of land advancing slowly into the sea. Thus all of the world’s great outie river-mouths – the fertile deltas that have helped foster human civilization since its birth – came into being at just about the same time. The same can’t be said for all the world’s outie belly-buttons. What can be said, though, is that innies and outies - both for rivers and people - are a small record of how we came to be. A huge thank-you to the following organizations, all working toward sustainable deltas, for sponsoring this video: the Belmont Forum, the Sustainable Deltas Initiative, the National Center for Earth-Surface Dynamics, the St Anthony Falls Laboratory of the University of Minnesota, and the DELTAS project. These organizations study deltas around the world, in particular how they’re threatened by human activities such as building dams, channelizing rivers, and climate change-induced sea-level rise. If we don’t pay attention, we might lose the landform that allowed us to become civilized in the first place.