Danube
"Danube" is an Old European river name derived from a Proto-Indo-European "*dānu". Other river names from the same root include the Dunaj, Dzvina/Daugava, Don, Donets, Dnieper, Dniestr, Dysna, Tana and Tuoni. In Rigvedic Sanskrit, "dānu" means "fluid, drop", in Avestan, the same word means "river". In the Rigveda, "Dānu" once appears as the mother of Vrtra, "a dragon blocking the course of the rivers." The Finnish word for Danube is Tonava, which is most likely derived from the word for the river in Swedish and German, "Donau". Its Sámi name "Deatnu" means "Great River". It is possible that "dānu" in Scythian as in Avestan was a generic word for "river": "Dnieper" and "Dniestr", from "Danapris" and "Danastius", are presumed to continue Scythian "*dānu apara" "far river" and "*dānu nazdya-" "near river", respectively.
Isar
One theory is that the name "Isar" comes from the hypothetical Indo-European root "*es" or "*is", which meant "flowing water" and later turned into a word for frozen water (hence English "ice", ) in Proto-Germanic. An older theory is that it comes from Celtic words and the name "Isar" is a construction of the Celtic stems "ys" "fast, torrential" and "ura" "water, river". According to another interpretation "ys" may mean "high" as well as "low", referring to the rapidly changing water level in the river Isar. In the ancient settlement area of the Celts several related river names can be found:It may be possible that the ancient name of the lower part of the river Danube, Ister, has the same source. Newer interpretations relate it to the Proto-Basque particle "*(w)its-", "water", also seen in similar but otherwise unexplained river names, such as Vézère, Vizela, Weser and Vistula.