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Strahler Stream Order animation: what looks like the silhouette of a tree with branches but no leaves, is actually a plan view of a large river network. Starting at the outermost, narrowest parts, each time two tributaries of the same order merge together the order of river size increases until it flows out into the sea.

Strahler stream order: Streams, rivers, the Amazon and how to organise them.

A clever means of categorizing the size of rivers. It does this by looking at joins of tributaries. Each time two tributaries of the same order join together then the river goes up in order. So the start of a stream from a spring is the outermost branch — first order. When two first orders join together, the river is classified as second order and so on.

The largest river by Strahler stream order classification is the Amazon at twelfth order. This means that at least two order 11 rivers had to merge together before the river emptied into the sea.

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