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.
Serbia
Almost all of Serbia's rivers drain to the Black Sea, by way of the Danube river. The Danube, the second largest European river, passes through Serbia with 588 kilometers (21% of its overall length) and represents the largest source of fresh water. It is joined by its biggest tributaries, the Great Morava (longest river entirely in Serbia with 493 km of length), Sava and Tisza rivers. One notable exception is the Pčinja which flows into the Aegean. Drina river forms the natural border between Bosnia and Herzegovina and Serbia, and represents the main kayaking and rafting attraction in both countries.