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.
Vienna Danube regulation
After repeated severe flooding, in 1810 Hofbau-Direktor Josef von Schemerl proposed regulating the river by creating a new river bed, but his plans were not realized. From 1870 to 1875 the Danube was regulated for the first time. On the left bank, a wide flood-prone area, the "Inundationsgebiet", was created with the dam Hubertusallee covering today's municipal districts of Floridsdorf and Donaustadt. The new main branch, including shipping, was wide, leaving the old river bed as the Old Danube. Additional major floods in the years 1897, 1899 and 1954, especially concerning the right bank of the Danube at "Handelskai" ("Trade pier"), proved that this 1875 regulation of the Danube was insufficient.