But rivers don't have umbilical cords, so why do they have innies and outies?

但是河流又没有脐带，那它们为什么会有凹凸的区别呢？

But in general, belly buttons have two basic different shapes: 'innies' and 'outies.'

但一般来说，肚脐有两种基本不同的形状：凹脐和凸脐。

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.

河流同样也有凹凸之分，不过不是肚脐，而是河口：在河流入海的地方陆地会向外延伸，或者向内弯曲。