Danube

The Danube is a river that was once a long-standing frontier of the Roman Empire and today connects 10 European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for 2,850 km, passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine before draining into the Black Sea. Its drainage basin extends into nine more countries. The largest cities on the river are Vienna, Budapest, Belgrade and Bratislava, all of which are the capitals of their respective countries. The Danube passes through four capital cities, more than any other river in the world. Five more capital cities lie in the Danube's basin: Bucharest, Sofia, Zagreb, Ljubljana and Sarajevo. The fourth-largest city in its basin is Munich, the capital of Bavaria, standing on the Isar River.

Source: Wikipedia