Socialist Federal Republic of Yugoslavia
Regarding rivers, the Danube was navigable throughout its entire course in Yugoslavia, linking the ports of Belgrade, Novi Sad, and Vukovar with Central Europe and the Black sea. Long stretches of rivers Sava, Drava and Tisza were also navigable.
Breg (river)
After 46.2 kilometres, the Breg merges with the Brigach in Donaueschingen to form the Danube - ""Brigach und Breg bringen die Donau zuweg"" ("Brigach and Breg bring the Danube together") as the well known rhyme goes. Since the Breg is not only the longest, but also the voluminous river of the Danube with a catchment area of 291.5 km², its source is considered hydrographically as the source of the Danube ("Donauquelle").