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.
Topography of Romania
After entering the country in the southwest at Bazias, the Danube travels some 1,000 km through or along Romanian territory, forming the southern frontier with Serbia and Bulgaria. Virtually all of the country's rivers are tributaries of the Danube, either directly or indirectly, and by the time the Danube's course ends in the Black Sea, they account for nearly 40 percent of the total discharge. The most important of these rivers are the Mureş River, the Olt River, the Prut, the Siret River, the Ialomiţa River, the Someş River, and the Argeş River. Romania's rivers primarily flow east, west, and south from the central crown of the Carpathians. They are fed by rainfall and melting snow, which causes considerable fluctuation in discharge and occasionally catastrophic flooding. In the east, river waters are collected by the Siret and the Prut. In the south, the rivers flow directly into the Danube, and in the west, waters are collected by the Tisza on Hungarian territory.