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.
Hydrology of Hungary
The Danube is the second longest river in Europe, after the Volga. Its upper flow is outside of Hungary, but its middle section starts at Devín, close to the Hungarian border. This section of the river stretches until the Iron Gate, between Serbia and Romania. The lower section of the river crosses the Romanian Plain and flows into the Black Sea through a delta. The Hungarian section of the river is long, from which forms the border between Hungary and Slovakia, from Rusovce to the mouth of the Ipeľ ().