Danube
The Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries, more than any other river in the world. Originating in Germany, the Danube flows southeast for , passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine before draining into the Black Sea. Its drainage basin extends into nine more countries. The Danube river basin is home to fish species such as pike, zander, huchen, Wels catfish, burbot and tench. It is also home to a large diversity of carp and sturgeon, as well as salmon and trout. A few species of euryhaline fish, such as European seabass, mullet, and eel, inhabit the Danube delta and the lower portion of the river.
European watershed
The divide continues northwards along the Albula Alps to Julier Pass, Albula Pass and Flüela Pass south of Davos between the catchment area of the Rhine, which empties into the North Sea via the Netherlands, and the Danube, which flows eastward emptying into the Black Sea. It enters Austria at Piz Buin in the Silvretta Alps running along the Arlberg massif and forming the border between the states of Tyrol and Vorarlberg. It reaches Bavaria, Germany via the crest of the Allgäu Alps and leaves the High Alps at the Adelegg range near Kempten.