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.
EV6 The Rivers Route
It usually runs along both sides of the Danube, though sometimes it runs only along one side so that you must cross the Danube a number of times during the course of the route. The Danube flows through very diverse landscapes, including mountainous terrain and passes through a few nature reserves. The route passes several mountains, towns and monasteries, artistic buildings and geological features. Where the current crosses great plains, the bike path often runs along high water dams with extensive views, beginning in Lower Bavarian, northern Austria (e.g. Marchfeld) and finally in Hungary.