Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries, more than any other river in the world.
Originating in Germany, the Danube flows southeast for 2,850 km (1,770 mi), passing through or touching the border of Austria,Slovakia,Hungary,Croatia,Serbia,Romania,Bulgaria,Moldova and Ukraine before draining into the Black Sea.
The river was known to the ancient Greeks as the Istros.
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 2,730 km (1,700 mi), passing through four capital cities (Vienna,Bratislava,Budapest and Belgrade) before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
The Danube river's drainage basin is 817,000 square kilometers in size.19 countries share the Danube River Basin,which makes it the world's most international river basin.