Danube basin

The Danube ( ; see also other names) is a river in Europe, the second-longest after the Volga in Russia. It flows through Central and Southeastern Europe, from the Black Forest of Germany south through the Danube Delta in Romania into the Black Sea. A large and historically important river, it was once a frontier of the Roman Empire. In the 21st century, it connects ten European countries, running through their territories or marking a border. Originating in Germany, the Danube flows southeast for , passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Mo

river delta in east-central Europe

canal in Bavaria, Germany which connects the Main and the Danube rivers across the European Watershed

navigable canal on the Danube river in Romania

river in Slovakia

canal

thumb|right|Aachtopf historical postcard, 1910 The Aachtopf () is Germany's biggest karst spring, south of the western end of the Swabian Jura near the town of Aach. It produces an average of 8,500 litres per second. Most of the water stems from the River Danube where it disappears underground at the Danube Sinkhole, north near Immendingen and about north near Fridingen. The cave system has been explored since the 1960s, but as of 2020 only a small part has been discovered due to a large blockage after a few hundred metres.

waterfalls in Muggendorf, Lower Austria

canal from Romania