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.
EuroVelo
Running from Saint-Nazaire on the mouth of the River Loire along that river eastward through France, EV6 passes over the border to Switzerland to Lake Constance and then on to Tuttlingen in Germany, where it begins its way down the Danube following the (Danube Cycle Route). It follows that river, Europe's second longest, through Germany, Austria, Slovakia, Hungary, Serbia, Bulgaria and Romania to the river's mouth at the Danube Delta. It then continues southwards to end in Constanța, on the Black Sea.
EV7 runs from the North Cape to Malta, will whisk you from the Arctic Circle in Norway, the land of the midnight sun, to island hopping in the Mediterranean.