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

Strahler stream order is a smart way of categorizing the size of rivers by tracking where tributaries join.

Each time two tributaries of the same order join together, the river rises in order.

So, the start of a stream from a spring is the outermost branch—it's first order. When two first-order tributaries join together, the river is classified as second-order, and so on.

The largest river by Strahler stream order classification is the Amazon, which is an incredible 12th-order river. This means that at least two 11th-order rivers had to merge together before the river emptied into the sea. Huge.