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.
Hydrology of Hungary
The Danube is the second longest river in Europe, after the Volga. Its upper flow is outside of Hungary, but its middle section starts at Devín, close to the Hungarian border. This section of the river stretches until the Iron Gate, between Serbia and Romania. The lower section of the river crosses the Romanian Plain and flows into the Black Sea through a delta. The Hungarian section of the river is long, from which forms the border between Hungary and Slovakia, from Rusovce to the mouth of the Ipeľ ().