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.
International Commission for the Protection of the Danube River
The commission became active in 1998. Since then, it has grown into one of the largest and most active international bodies of river basin management expertise in Europe. The commission deals not only with the Danube itself, but also with the whole Danube River Basin, which includes more than 300 tributaries and the ground water resources.