Danube
Classified as an international waterway, it originates in the town of Donaueschingen, in the Black Forest of Germany, at the confluence of the rivers Brigach and Breg. The Danube then flows southeast for about , passing through four capital cities (Vienna, Bratislava, Budapest and Belgrade) before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
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.