River mouths are a lot like belly buttons. Okay, they're really not, but in the sense that humans can either have innie or outie belly buttons depending on how the connection with the umbilical cord was cut, when the mouth of a river connects with the ocean, the land will either poke out to form river deltas, or bend inwards. But why does this battle between land and sea result in such different forms? The latest episode of MinuteEarth explains how the spoils of this epic skirmish are the fertile river deltas we have today.
When two opposing forces - the land and the sea - meet, something eventually has to give. Increases in sea level could see it rise above the land, or if the land is affected by severe erosion, it can sink below the existing sea level. Another option is that the sea level drops below the land, or the land itself is built or lifted up somehow to above the sea level.
Pretty simple, right? It is, until rivers get involved. During the last ice age - about 120,000 years ago - sea levels fell by about 120 metres. This caused the rivers to cut deeper and deeper valleys into the icy coast to meet the falling seas, explains the video.
That is until about 18,000 years ago, when all that ice started to melt away, and this caused significant increases in sea level. All that extra ocean ended up flowing up into the valleys, creating giant estuaries, and what Henry Reich calls the coastal version of 'innies' above.
But sea levels don't stay the same over thousands of years - they change, and this has seen the formation of different interactions between land and sea, land and rivers, and rivers and the sea. About 7,000 years ago, the warming slowed down, and sea levels stopped increasing. This allowed some coastlines to gain back ground.
As the land and river mouths are once again exposed, something fascinating happens to create the complex, snaking avenues of fresh water called deltas that slowly wind their way into the ocean. You'll have to watch the latest episode of MinuteEarth to find out what, but let's just say that human civilisation would not be close to what it is today without this amazing physical phenomenon.