The river originates in the Black Forest in Germany as the much smaller Brigach and Breg rivers which join at the German town of Donaueschingen. After that it is known as the Danube and flows southeastward for a distance of some 2850 km (1771 miles), passing through four Central and Eastern European capitals, before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
Known to history as one of the long-standing frontiers of the Roman Empire, the river flows through or acts as part of the borders of ten countries: Germany (7.5%), Austria (10.3%), Slovakia (5.8%), Hungary (11.7%), Croatia (4.5%), Serbia (10.3%), Bulgaria (5.2%), Moldova (0.017%), Ukraine (3.8%) and Romania (28.9%).
On October 7, 2010, a toxic red sludge that burst out of a Hungarian aluminum factory's reservoir entered the Danube, threatening an ecological disaster. The pH content of the red sludge entering the Danube had been reduced to the point where it was unlikely to cause further environmental damage. It had been tested earlier at a pH level of 13 and now was down under 10, and no dead fish had been spotted where the slurry was entering the Danube