The Danube rises in two sources (the Brege and Brigach rivers) in the Black Forest, SW Germany, and flows NE across S Germany past Ulm to Regensburg, where it turns SE to enter Austria at Passau. It continues SE through Upper and Lower Austria, past Linz and Vienna. It then forms the border between Slovakia and Hungary from Bratislava to Szob. At Szob, the Danube turns south and flows across the Great Alföld (plain) of central Hungary, past Budapest. After forming the northern two thirds of the Croatia-Serbia border, it enters Serbia above Belgrade, turns southeast, then east, and flows through narrow gorges, forming part of the Serbia-Romania border. The Iron Gate gorge, site of a hydroelectric dam, is there; the Sip Canal bypasses rapids in the gorge. After passing the Iron Gate, the Danube broadens again and forms most of the Romania-Bulgaria border before swinging north near Silistra and passing through E Romania to Galaţi, where it divides into an expansive (c.1,000 sq mi/2,590 sq km) delta before entering the Black Sea. The northernmost branch of the delta runs along the frontier between Romania and Moldova and Ukraine. The central, canalized branch is the main shipping route. The Danube receives more than 300 tributaries, notably the Inn, Drava, Tisza, Sava, and Prut.