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.
Chain boat navigation
However, fast currents when rivers were in spate could also be problematic for chain boats. Depending on the nature of the river bed, strong movements of sediment could lead to ements to aggradation, thus covering the chain with gravel and stones. A river bed with lots of ledges or large boulders – such as on some sections of the Danube – caused the chain to become snagged and was a major hindrance to chain boats.