Danube

The Danube (pronounced /ˈdænjuːb/ DAN-ewb in English) is the longest river in the European Union and Europe's second longest river after the Volga. It is notable for being classified as an international waterway.
The river originates in the Black Forest in Germany as the much smaller Brigach and Breg rivers which join at the German town of Donaueschingen, after which it is known as the Danube and flows eastward for a distance of some 2850 km (1771 miles), passing through four Central and Eastern European capitals, before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
Known to history as one of the long-standing frontiers of the Roman Empire, the river flows through or alongside the borders of ten countries: Germany (7.525), Austria (10.325), Slovakia (5.825), Hungary (11.725), Croatia (4.525), Serbia (10.325), Bulgaria (5.225), Moldova (0.01725), Ukraine (3.825) and Romania (28.925).

kilometri.bg/en/view/the-danube-river
ancientbulgaria.bg/listings/danubian-border