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 primaeval Danube first silted up the basin of the Pannonian Sea which now forms the Eastern Slovak Lowland on the left bank of the river and the Little Hungarian Plain on the right bank. It flowed towards the south into Transdanubia and then followed the present-day course of the Drava into an interior lake system in the south of the present Great Plain. Later the Danube was diverted to the east by tectonic uplift, finding an outlet through the Visegrád gap. During the Quaternary the Great Plain continued to sink, meanwhile the Börzsöny and the Visegrád mountains were rising. The result was the gorge, or a deep terraced valley cut through the mountains by the Danube at what is now the Danube Bend at Visegrád, between Esztergom and Vác. Once through the gorge, the Danube fanned out into the Great Hungarian Plain, depositing alluvial cones. The present north-south course was formed by the river in the late Pleistocene.