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.
Havel
The Havel is navigable from the Mecklenburg Lake District to its confluence with the Elbe. Whilst its upper reaches carry little other than leisure traffic, further downstream it provides an important link in the German commercial waterway network, carrying traffic from the Rhine and the North Sea to Berlin and Poland.