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.
European watershed
Within the Alpine foothills it runs north of the Schussen catchment area of Lake Constance towards the sources of the Brigach and Breg headwaters of the Danube in the Black Forest. It then follows the main ridge of the Swabian and Franconian Jura with the crossing of the Rhine–Main–Danube Canal northeastwards up to the Fichtelgebirge and southeastwards along the Bohemian Forest, forming the border between Germany and the Austrian Mühlviertel and Waldviertel regions with the Czech Republic, where the large Bohemian basin is drained by the Elbe River. It again turns northeastwards along the Bohemian-Moravian Highlands-Javořice Highlands to the Śnieżnik Mountains of the Eastern Sudetes at the border with Poland, where the tripoint of Orlice, Elbe (North Sea) and Morava, Danube (Black Sea) and Eastern Neisse, Oder (Baltic Sea) is located at the peak of Trójmorski Wierch (Klepý, Klappersteine, Three seas peak, 1144 m above sea level).