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.
Breg (river)
The Breg rises at a height of , six kilometres northwest of Furtwangen. Its source, which is near St. Martin's Chapel and is also called the source of the Danube or "Donauquelle", is protected as a natural monument. It is located about 100 metres south-east of the Rhine / Danube watershed (the great European Watershed). Beyond that, but only about 900 metres away is the source of the Elz, which flows in the same longitudinal valley but initially in the opposite direction, to the north, and later flows into the Rhine.
In its upper section, known as the "Katzensteig", the Breg valley is clearly a result of glaciation, with a strikingly low gradient and landscape characterized by large Black Forest houses. Between the towns of Furtwangen and Vöhrenbach, the Breg flows eastwards through a broader and somewhat more densely populated valley, then in a generally southeasterly direction through a solitary forest valley, accompanied by a road and the route of the former Breg Valley Railway, which is now a bicycle track. Shortly before Hammereisenbach-Bregenbach, the Breg is joined by the Linach just below the Linach Dam and, in the village, by the Hammerbach, a short tributary, but the most important thanks to its two large headstreams. Near Bräunlingen, the Breg meets the Röthenbach stream at the Kirnbergsee. Here the Breg leaves the Black Forest and continues via Hüfingen to Donaueschingen, through wide, open countryside on the plateau of the Baar.