Danube
Classified as an international waterway, it originates in the town of Donaueschingen, in the Black Forest of Germany, at the confluence of the rivers Brigach and Breg. The Danube then flows southeast for about , passing through four capital cities (Vienna, Bratislava, Budapest and Belgrade) before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
Danube Sinkhole
In the long run, today's upper Danube will probably be redirected entirely into the Radolfzeller Aach, and thus to the Rhine. This would mean that other tributaries that are currently insignificant would become the new headwaters of the Danube. The Danube has experienced a similar stream capture further upstream, during the Würm glaciation, when the "Feldberg-Danube" was captured by the Wutach at the Wutach Gorge near Blumberg.