It has been proven that many rivers, especially those which flow into Vistula and the Odra river i n Poland as well as in neighbouring countries have suspicioulsy celtic origins. Most cannot be derived from any kniown European language. Therefore, those must have been taken over from some previous population, dwellers of those lands. Which means that some tribes must have endured the wandering of the tibes and the settler were used by the Slavs as informants, or perhaps even guides. Whatever happened to them? What were propotions of Slavic and native populations? Were only the names of Rivers given to them?