with outies. Rivers also have innies and outies – not belly buttons, but mouths: where rivers

河流也有凹凸之分─不是肚臍，而是河口

flow into the sea, the land either pokes out or bends inward. But rivers don’t have umbilical

當河流入海，陸地可能外擴或內縮

cords, so why do they have innies and outies? Well, coasts are the front lines between two

但河流又沒有臍帶，為啥會有凹凸之別?

opposing forces – land and water. In order for the ocean to invade the land, sea level

當海水侵蝕陸地海岸是兩種力量對抗的前線─陸與水 當海水侵蝕陸地

either has to come up, or the land has to sink down or be eroded1 away. And in order for

海平面會上升，陸地會下降或被沖刷掉

the land to advance into the ocean, sea level either has to drop, or the land has to build

當陸地推向海洋，海平面會下降，土地會堆積(或抬升)

(or be lifted) up. Obviously, if sea level drops and then rises

顯然，如果海平面下降而後上升

back again, there’s no net gain on either side – but things get more complicated when

最終兩邊都會回到原點─但河流的參戰使事情更複雜。

a river joins the battle. For example, during the last ice age, sea levels fell by over

比方說在上次冰河期，海平面下降超過120公尺

120 meters, and rivers cut deeper and deeper valleys to reach the falling seas. Then, about

河流切割出越來越深的山谷以降至海面。

18,000 years ago, warming temperatures began to melt the ice, and the rising seas flooded

但到了18,000年前，氣候回暖使冰熔化

river valleys around the world, creating giant estuaries and giving us the

此時海水上升淹沒河谷

innie-riddled coastlines we have today. But when the steady landward march of the

在世界各地產生巨大河口與凹陷的海岸線。

seas finally began to slow, about 7,000 years ago, the coastlines around the mouths of some

但當海面上升的趨勢於約7,000年前趨緩

rivers began to gain back some ground. The key factor was the sediment the

某些河流開始奪回出一些土地

river dropped as its current slowed at the entrance to the sea. Where the sediment supply

關鍵原理在於砂土在入海時受阻而減緩

was big enough and the ocean was calm enough, the dropped dirt piled up, eventually forming

若砂土夠多且海洋平靜，它們會開始堆積

new land that both lengthened the river and divided it in two. Dirt continued to drop

最終產生新陸地甚至把河流一分為二。

out and build up at the mouths of both channels, splitting the river again...and again...and

砂土會把兩個河口分半再分半、再再分半......

again, creating a new lobe of land advancing slowly into the sea. Thus all of the world’s

堆積出一片新陸地緩緩深入海洋

great outie river-mouths – the fertile deltas that have helped foster human civilization

因此世上許多大型凸出河口，如尼羅河或長江，

since its birth – came into being at just about the same time.

孕育了許多文明─他們幾乎同時形成。

The same can’t be said for all the world’s outie belly-buttons. What can be said, though,

當然這不適用於世界上所有凸肚臍的人，我們只是說

is that innies and outies - both for rivers and people - are a small record of how we

凹的或凸的─不管是河口或肚臍─是我們出生的小小紀念

came to be. A huge thank-you to the following organizations,

以下組織非常感謝您的收看： 致力於保護河口三角洲，並贊助此影片的包括

all working toward sustainable deltas, for sponsoring this video: the Belmont Forum,

Belmont Forum、Sustainable Deltas Initiative、National Center for Earth-Surface Dynamics、

the Sustainable Deltas Initiative, the National Center for Earth-Surface Dynamics, the St

St Anthony Falls Laboratory of the University of Minnesota與DELTAS project。

Anthony Falls Laboratory of the University of Minnesota, and the DELTAS project. These

這些組織研究世界各地的三角洲，

organizations study deltas around the world, in particular how they’re threatened by

尤其是人類活動的威脅如水壩、更改河道 與氣候變遷導致海平面上升。

human activities such as building dams, channelizing rivers, and climate change-induced sea-level

若我們再不注意，將會失去我們文明的起源地。

rise. If we don’t pay attention, we might lose the landform that allowed us to become

civilized in the first place.