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.
Danube Commission (1948)
When the treaty was adopted, Yugoslavia had already been expelled from the Cominform, the political grouping of all the Communist parties in the Soviet bloc. Yet it still voted down the line with the other non-Western countries, nearly 200 miles of the Danube flowing through its territory and the only navigable channel through the Iron Gate being on the Yugoslav side of the Romanian border. Nevertheless, when the new commission organized its staff, the Yugoslavs were offered only four minor posts out of sixty permanent appointments. The Josip Broz Tito government refused them all.