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  • The world is divided into two kinds of people: those with innie belly buttons, and those

  • with outies. Rivers also have innies and outiesnot 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 forcesland 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 sidebut 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-mouthsthe fertile deltas that have helped foster human civilization

  • since its birthcame 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 theyre 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.

The world is divided into two kinds of people: those with innie belly buttons, and those

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