"The non-Indo-Aryan nature of the terms and names noted earlier also has to be juxtaposed with the fact that the place and river names in northern India are almost all Indo-Aryan. Place and river names are, to my mind, the singlemost important element in considering the existence of a substratum. Unlike people, tribes, material items, flora and fauna, they cannot relocate or be introduced by trade, etc. (although their names can be transferred by immigrants). Place names tend to be among the most conservative elements in a language. Moreover, it is a widely attested fact that intruders into a geographical region often adopt many of the names of rivers and places that are current among the peoples that preexisted them, even if they change the names of others (i.e. the Mississippi river compared to the Hudson, Missouri state compared to New England). With this in mind, it is significant that there are very few non-Indo-Aryan names (and almost none whose etymologies are completely uncontested) for rivers and places in the North of the Indian subcontinent, which is very unusual for migrants intruding into an alien language-speaking area. All the place names in the Rgveda, which are few in number, are Indo-Aryan, or at least sanskritized..."