Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are 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").