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.
Geography of Austria
The Danube has its source near Donaueschingen in southwestern Germany and flows through Austria before emptying into the Black Sea. It is the only major European river that flows eastwards, and its importance as an inland waterway has been enhanced by the completion in 1992 of the Rhine-Main-Danube Canal in Bavaria, which connects the Rhine and Main rivers with the Danube and makes barge traffic from the North Sea to the Black Sea possible.