arvistro posted the map of Baltic hydronyms by linguist Vasiliev already.
Article published in journal of Linguistics by Vasiliev in 2015
In short, there are maximum number of Baltic hydronyms till Ilmen lake (outer shaded area). Minimum number of hydronyms till middle Volkhov river (inner shaded area) . There enclaves with high concentration of Baltic hydronyms. There are no Baltic hydronyms further north ie in Karelia and Archangel region of Russia.
Article published in journal of Linguistics by Vasiliev in 2015
The problems of studying the Baltic origins of hydronyms on the territory of Russia
Валерий Л. Васильев
Abstract
The article raises a wide range of issues related to the ancient presence of Balts in Russian hydronymy. The entire eastern half of the vast Old Baltic ethno-historical area overlapped with regions of European Russia. The names of rivers and lakes are the main source that allows to ascertain the extent of their ancient settlement as well as some features of their language. This paper provides an overview of the history of the study of hydronyms of Baltic origin on the territory of Russia, including the criticism of certain previous works, and briefly highlights a number of issues, among which are: 1) the refinement of the South-eastern, Eastern and especially North-eastern borders of the Old Baltic range, 2) the correlation of the Baltic linguistic elements with the Finnish and the Iranian elements, 3) the relationship of Baltic hydronymy to archaeological cultures, localizable in European Russia, 4) the relative density of the Baltic stratum in different Russian regions, 5) the optimization of techniques related to searching and confirmation of Baltic water names. Maps are provided to show the North-eastern flank of the Baltic hydronymic areal and the maximum geographical extent of Eastern Baltic hydronymy.
The article is in Russian: http://revije.ff.uni-lj.si/linguistica/article/view/6308
In short, there are maximum number of Baltic hydronyms till Ilmen lake (outer shaded area). Minimum number of hydronyms till middle Volkhov river (inner shaded area) . There enclaves with high concentration of Baltic hydronyms. There are no Baltic hydronyms further north ie in Karelia and Archangel region of Russia.