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.
Regen (river)
The Regen (; ) is a river in Bavaria, Germany, and a left tributary of the Danube, at Regensburg, Germany. The source of its main headstream, the Great Regen ("Großer Regen"), is in the Bohemian Forest on the territory of the Czech Republic, near Železná Ruda. The river crosses the border after a few kilometres, at Bayerisch Eisenstein. The name in German evolved from the name in Latin, but its meaning is unknown. The Romans called the river variously "Regana" (feminine gender), "Reganus" (masculine), and Reganum (neuter).