DANUBE (Ger. Donau, Hungarian Duna, Rumanian Dun area, Lat. Danubius or Danuvius, and in the lower part of its course Ister), the most important river of southern Europe. Ris ing in the Black Forest mountains and emptying into the Black sea, it receives tributaries on the right bank from the eastern Alps, the Dinaric Alps and the Balkan mountains, and on the left bank from the Frankischer Jura, Bohmer Wald, Bohmisch-Mahrisch Hohe, the Carpathians and the Transylvanian Alps. It is 1,750 m. long, drains an area of 320,200 sq.m. and is the most important river of Europe as regards the volume of its outflow, but inferior to the Volga in length and drainage area. The river is first called the Danube at Donaueschingen in the Black Forest, where three streams, the Brigach, the Brege and a smaller stream meet at an altitude of 2,187 feet. It is navigable for special river craft below Ulm (height 1,505 ft. above sea level), and it is fed by at least 300 tributaries many of which are themselves mighty streams.
The river can be divided into three sections ; the upper course, above Bratislava, the middle course, between Bratislava and the Iron Gates and the lower course, below the Iron Gates.
The direction of the river in its upper course is determined by the structure of the Alpine foreland. Rising in the crystalline rocks of the Black Forest it flows eastward across a narrow belt of Jurassic rocks to Sigmaringen and from thence to Regensburg along the northern edge of the Swiss, plateau (see ALPS), its bed being in the soft Molasse (Upper Oligocene and Miocene rocks) and its direction following the so-called Danube Fault, which passes from Schaffhausen to Regensburg. Below the latter town the river is deflected south-eastward by the Bohemian massif and flows in part upon the crystalline rocks of the latter and in part upon the Molasse, but at Krems it turns eastward across the Molasse and after passing Vienna it flows through the gap which separates the eastern Alps from the Carpathians. The valley of the Danube above Sigmaringen is narrow, the scenery being wild and beautiful, especially above Tuttlingen, where castles crown every possible summit on the neighbouring hills. Below Imrend ingen much water escapes by subterranean fissures into the river Ach, a tributary of the Rhine. After it is joined on the right bank by the Iller, which tributary rises in the Algauer Alps, the Danube attains a width of 78 yd. and an average depth of 3 ft. 6 in. and becomes navigable downstream for specially constructed craft of 10o tons. At Donauworth (height 1,33o ft.) it receives the Lech, which tributary rises near the Iller and flows in a direction parallel to it, whilst at Regensburg (height 949 ft.) the Danube receives on the left bank the river Naab which rises in the Fichtel Gebirge. Below Regensburg, at Deggendorf, it is joined by the Isar, on the banks of which stands Munich. The upper course of the Danube lies in German territory, rising in Baden and flowing through Wurttemberg and Bavaria. At Passau (height Boo ft.) it enters Austria and approximately 1 oo m. of the north-western boundary of that country is formed by the Danube, its right bank tributary the Inn, and the Salzach which flows into the Inn on its right bank. The rivers Isar, Inn and Salzach drain a large portion of the eastern Alps and have many important towns on their banks. The Inn joins the Danube at Passau.