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. 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 .
Navigation is impeded by ice in winter and by seasonally varying water levels.
Romanian efforts in the 1980s to drain land in the delta for agriculture damaged Europe's largest wetlands, which are now being rehabilitated. During the Kosovo crisis in 1999, NATO air strikes destroyed bridges across the river in Serbia, obstructing river commerce. As Rome declined, the Danubian plains for centuries attracted invading hordes-Goths, Huns, Avars, Magyars, Pechenegs, Cumans, Mongols, and others. In the 19th cent. Germany repudiated the internationalization in 1936 and in 1939-40 forced both the navigation and international commissions to dissolve.