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.
Danube
Since the Swabian Alb is largely shaped of porous limestone, and since the Rhine's level is much lower than the Danube's, today subsurface rivers carry much water from the Danube to the Rhine. On many days in the summer, when the Danube carries little water, it completely oozes away noisily into these underground channels at two locations in the Swabian Alp, which are referred to as the Donauversickerung (Danube Sink). Most of this water resurfaces only south at the Aachtopf, Germany's wellspring with the highest flow, an average of , north of Lake Constance—thus feeding the Rhine. The European Water Divide applies only for those waters that pass beyond this point, and only during the days of the year when the Danube carries enough water to survive the sink holes in the Donauversickerung.