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.
New Danube
The New Danube flows parallel to the river for approximately through the Vienna metropolitan area. It diverges from the main channel in Langenzersdorf in Lower Austria (northwest of the city), flows through the 21st district, and rejoins it at the Danube-Auen National Park in the 22nd district (in the southeast). The channel is about wide; the island varies between in width. The current course of the New Danube and the site of the Danube Island were previously a broad landscape of meadows set aside as a floodplain in the first effort to manage Danube floods, in 1868–75. The left, northern bank of the New Danube is protected by the Hubertus Dam (Marchfeld Protective Dam), completed in 1875.