A river structure is best shown as a tree with the main stream as the trunk and the tributaries as branches. This lends itself to "grading" the streams. The main stream that enters the sea is given grade 1. Its first tributaries are grade 2. Their tributaries are grade 3 and so on.Normally the grade 1 stream will have the oldest name. Higher grade streams will have newer names and those furthest from the mouth will very often be nameless. As we climb away from the estuary and up in grade the likelihood of a small local affluent having no name or having been named only in relatively recent periods increases.