Rivers flowing into the English Channel and the North Sea drain the modern Departments of Somme, Pas-de-Calais and Nord.
Rivers often bear the oldest names
and in this region there are many with names predating the use of
the Celtic language.
A river structure is best shown as a tree with the trunk being the main stream and the branches the tributaries.
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. 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.
River names are not always found attached to rivers. Just as today a settlement can be shown to have been named for the river that waters it, we can assume that many others were once so named.
Thus we can hope to find river names buried in
settlement names although the stream in question has long been
renamed or its name forgotten.