The Danube is conventionally taken to be formed by the confluence of the two streams Brigach and Breg just east of Donaueschingen.
The source of the Donaubach, which flows into the Danube, in Donaueschingen is often referred to as the source of the Danube. Hydrologically, the source of the Danube is the source of the Breg as the larger of the two formative streams which rises near Furtwangen.
There was an active rivalry between the municipalities of Furtwangen and Donaueschingen for the claim of being home to the "official" source of the Danube since the 1950s, sometimes with the involvement of the government of the state of Baden-Württemberg.
Wikipedia https://en.wikipedia.org/wiki/Source_of_the_Danube
Official website http://www.donaueschingen.de/donauquelle
Address 1 An der Stadtkirche, Donaueschingen 78166, Germany
Coordinates 47°57'6.617" N 8°30'8.964" E