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–Oder Canal
The Holy Roman Emperor Charles IV had envisioned a waterway capable of taking ships from the Oder to the Danube in the 14th century. Records of further plans of such a waterway stem from the 19th century. During the Nazi era, the idea was reborn and the project was put into motion. The project would connect the Oder to the Danube through the Moravian region of the Czech Republic, stretching 320 kilometers and spanning an elevation of 124 meters.