The Danube flows through Vienna, Bratislava, Budapest and Belgrade. It is Europe’s second-longest river, after the Volga, and runs for about 2,850 kilometres from the Black Forest region of Germany to the Black Sea.
The river passes through or along the borders of ten countries, more than any other river in the world. Its basin extends across a much larger area, linking landscapes from Central Europe to the Balkans. The Danube has long served as a trade route, a political boundary and a cultural connection between different regions.
Near its mouth, the river forms the Danube Delta, a large wetland shared mainly by Romania and Ukraine. The delta is important for birds, fish and other wildlife. Although the Rhine is another major European waterway, it does not pass through the four capital cities named in the question.