Danube River, German Donau, Slovak Dunaj, Hungarian Duna, Serbo-Croatian and Bulgarian Dunav, Romanian Dunărea, Ukrainian Dunay , river, the second longest in Europe after the Volga. It rises in the Black Forest mountains of western Germany and flows for some 1,770 miles (2,850 km) to its mouth on the Black Sea. Along its course it passes through 10 countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova, and Ukraine.
The Danube played a vital role in the settlement and political evolution of central and southeastern Europe. Its banks, lined with castles and fortresses, formed the boundary between great empires, and its waters served as a vital commercial highway between nations. The river’s majesty has long been celebrated in music. The famous waltz An der schönen, blauen Donau (1867; The Blue Danube), by Johann Strauss the Younger, became the symbol of imperial Vienna. In the 21st century the river has continued its role as an important trade artery. It has been harnessed for hydroelectric power, particularly along the upper courses, and the cities along its banks—including the national capitals of Vienna (Austria), Budapest (Hungary), and Belgrade (Serbia)—have depended upon it for their economic growth.