List of tributaries of the Danube
The Danube is Europe's second-longest river. It starts in the Black Forest in Germany as two smaller rivers—the Brigach and the Breg—which join at Donaueschingen, and it is from here that it is known as the Danube, flowing generally eastwards for a distance of some , passing through several Central and Eastern European capitals, before emptying into the Black Sea via the Danube Delta in Romania.
Regen (river)
The Regen (; ) is a river in Bavaria, Germany, and a left tributary of the Danube, at Regensburg, Germany. The source of its main headstream, the Great Regen ("Großer Regen"), is in the Bohemian Forest on the territory of the Czech Republic, near Železná Ruda. The river crosses the border after a few kilometres, at Bayerisch Eisenstein. The name in German evolved from the name in Latin, but its meaning is unknown. The Romans called the river variously "Regana" (feminine gender), "Reganus" (masculine), and Reganum (neuter).