Abstract—: The faunal, molecular, and palaeogeographic data that make it possible to discuss where and when aquatic animals could have crossed the watershed divide between the basins of the Caspian Sea and the Arctic Ocean are discussed. The freshwater fauna of Europe and northern Asia were largely similar in the Pliocene, while Pliocene fauna in Siberia went almost totally extinct during the Pleistocene glaciations. In interglacial periods, cold-water freshwater organisms colonized the Volga River basin, most likely during dispersal events from the basins of the Pechora and Ob’ rivers. Marine animals might have found their way into the Caspian Sea with saline waters arriving from the Arctic Ocean in the late Pliocene and early Pleistocene epochs. There was probably no possibility to cross the watershed divide during the last glacial period. After this period had ended, freshwater organisms, including mostly warm-water groups, entered the basins of the Onega, Northern Dvina, and Ob’ rivers as the watershed divide shifted to another location. Similar expansions currently take place due to human-mediated introduction and dispersal events through the channels connecting the Volga and Northern Dvina river basins.