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.
Topography of Romania
The Danube is by far Romania's most important river, not only for transportation, but also for the production of hydroelectric power. One of Europe's largest hydroelectric stations is located at the Iron Gates, where the Danube surges through the Carpathian gorges. The Danube is an important water route for domestic shipping, as well as international trade. It is navigable for river vessels along its entire Romanian course and for seagoing ships as far as the port of Brăila. An obvious problem with the use of the Danube for inland transportation is its remoteness from most of the major industrial centres. Moreover, marshy banks and perennial flooding impede navigation in some areas.