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.
Donau-Ries
In the south the district is crossed by the Danube, which runs from west to east. It is joined by the Lech river coming from the south. North of the river there is a hill chain, the so-called Riesalb connecting the mountain chains of the Swabian Alb in the west and the Frankish Alb in the east. Further north there is the Nördlinger Ries, a huge depression, which is in fact an ancient impact crater, caused by a meteorite some 14.8 million years ago.