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.
Ferenc Reitter
Reitter planned the route to follow a major swampy branch of the Danube river, which swings eastward in a large arch from Danube to Danube over a distance of 4.141 meters. The canal passed through a suburban district of narrow streets and, at the time, Reitter also considered making the backwater into a navigable canal to improve the air quality and lessen pollution.