Short answers, pulled from the story.
The Danube flows 2,850 kilometres from Donaueschingen in Germany's Black Forest to the Black Sea via the Danube Delta. It passes through or borders ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine.
Four national capitals sit on the Danube: Vienna (Austria), Bratislava (Slovakia), Budapest (Hungary), and Belgrade (Serbia). No other river in the world flows through as many national capitals.
The Danube Delta is the largest river delta in the European Union, with an approximate surface area of 4,152 square kilometres, most of which lies in Romania's Tulcea county. It became a UNESCO World Heritage Site in 1991 and hosts over 300 species of migratory birds, including the endangered pygmy cormorant.
The Rhine-Main-Danube Canal is a roughly 171-kilometre artificial waterway completed in 1992 that links the Danube at Kelheim with the Main at Bamberg. Together with the Danube, it creates a 3,500-kilometre trans-European waterway running from Rotterdam on the North Sea to Sulina on the Black Sea.
Danube is an Old European river name derived from the Celtic words 'Danu' or 'Don,' both names for Celtic gods, rooted in the Proto-Indo-European word deh2nu. The same root gave names to many other European rivers including the Don, Dnieper, and Dniester. In Rigvedic Sanskrit, danu means 'fluid' or 'dewdrop,' and in Avestan the same word simply means 'river.'
Six species of sturgeon once populated the Danube basin, but all are now threatened. Sturgeons had been commercially exploited since the 5th century BCE. The construction of Iron Gates I in 1974 and Iron Gates II in 1984 blocked migration routes without fish passage facilities, dramatically reducing spawning areas. The European sea sturgeon has largely or entirely disappeared from the river.