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.
Donauinsel
The Donauinsel (Danube Island) is a long, narrow island in central Vienna, Austria, lying between the Danube river and the parallel excavated channel "Neue Donau" ("New Danube"). The island is in length, but is only wide. The New Danube waterway is practically an elongated (swimming) lake, technically a diluvian bed. It has been described as an "autobahn for swimmers", due to its use by people commuting to work.