The River-Names of Europe — Robert Ferguson

The first wave of Asian immigration that swept over Europe gave names to the great features of nature, such as the rivers, long before the wandering tribes that composed it settled down into fixed habitations, and gave names to their dwellings and their lands. The names thus given at the outset may be taken therefore to contain some of the most ancient forms of the Indo-European speech. And once given, they have in many, if not in most cases remained to the present day, for nothing affords such strong resistance to change as the name of a river.