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.
Yantra (river)
The Yantra () is a river in northern Bulgaria, a right tributary of the Danube. It is long (the second longest Bulgarian tributary of the Danube, after Iskar) and has a watershed of .
The Yantra has its source from the northern foot of Hadzhi Dimitar Peak in Central Stara Planina, at . In its upper course, it is often called "Etar" (Етър), its older name. The river flows into the Danube close to Svishtov.