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)
The Danube Commission is concerned with the maintenance and improvement of navigation conditions of the Danube River, from its source in Germany to its outlets in Romania and Ukraine, leading to the Black Sea. It was established in 1948 by seven countries bordering the river, replacing previous commissions that had also included representatives of non-riparian powers. Its predecessor commissions were among the first attempts at internationalizing the police powers of sovereign states for a common cause.