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
In the 19th century, the Danube was an important waterway but was, as "The Times" of London put it, "annually swept by ice that will lift a large ship out of the water or cut her in two as if she were a carrot."