What river flows through Vienna and Belgrade?

The Danube (known by various names in other languages) is the second-longest river in Europe, after the Volga in Russia. It flows through much of Central and Southeastern Europe, from the Black Forest into the Black Sea. A large and historically important river, it was once a frontier of the Roman Empire and today connects ten European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for 2,850 km (1,770 mi), passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine.

Among the many cities on the river are four national capitals: Vienna, Budapest, Belgrade and Bratislava. Its drainage basin amounts to 817 000 km² and extends into nine more countries.