Danube (Lat. Danubius; in the lower course, Ister; Ger. Donau; Hung. Duna; Slav. Dunaj), the second river of Europe, inferior only to the Volga, has its origin in the Brege and Brigach, two mountain-streams rising in the Black Forest, in Baden, and uniting at Donaueschingen, 2264 feet above sea-level. The Danube has a total length, including windings, of 1740 miles, and drains an area estimated at 315,000 sq. m. It flows first SE. to Gutmadingen, and then NE. to Ulm (1519 feet above sea-level), which may be taken as the limit of its upper course. A few miles above Ulm it receives on its right bank the Iller, from which point it is navigable for boats of 100 tons. At Regensburg (Ratisbon) the river reaches its most northerly point (49° 2'), and from thence its course is generally SE. to the northern frontier of Bulgaria. Between Ulm and Passau, where it leaves German territory, it receives, among other tributaries, the three large alpine streams, the Lech, Isar, and Inn, on the right, and the Altmühl and Regen on the left bank. At Passau its width is 231 yards, and its depth 16 feet. The crystalline rocks of the Bohemian Forest mountains follow the stream into Austria, and as far as Aschach its banks are wild and picturesque. It flows E. to Presburg with little variation of course, receiving the Ens from the S., and the March or Morava from the N.; and it passes from the Austrian dominions through an opening, called the Carpathian Gate, between the Leitha chain and the Lesser Carpathians, where its width is 320 yards, and its depth 20 feet. Between Presburg and Komorn the stream divides, and forms the Great and Little Schütten, or alluvial islands. Near Waitzen it turns directly S., and enters upon the Hungarian plain, a vast sandy alluvial flat, in which it is continually forcing new channels and silting up old ones; it maintains this course for 230 miles, receiving from the N. the Waag and the Gran, while the Drave from the W. adds considerably to its volume.
After this last accession, the Danube turns again SE., and, increased by the waters of the Theiss and Temes from the N., sweeps past Belgrade, where it is joined by the Save, and forms the boundary between Hungary and Servia. At Semlin, near Belgrade, it is 1706 yards wide and 46 feet deep, but the width is greatly contracted by spurs of the Transylvanian and the Servian mountains for 75 miles beyond Ujpalanka. Within this stretch are eight distinct rapids, shut in by lofty walls, and strewn with rocky shoals of limestone, crystalline schists, and granite. The lower Klissura is the most strikingly picturesque of these, but the most difficult passage is the shortest (1½ mile) of the eight—the 'Iron Gate,' properly so called, below Orsova, where the middle course of the river ends. Here the stream has a breadth of only 129 yards, and the piled-up waters attain a depth of 28 fathoms; ledges of rock lift their tooth-like points above the surface, and all around a seething stretch of whirlpools, cataracts, eddies and counter-eddies, combines with the river's rapid fall to present a serious and formerly impassable obstacle to navigation. Many attempts have been made to improve the bed here, which, under article 57 of the Berlin Treaty, Austria bound herself to clear; but prior to 1889 little had been effected: only when a depth of at least 8 feet covered the obstructions could steamers drawing 5 feet make the passage. From here the Danube enters on the Wallachian depression, and flows in a wide stream, through a district fertile indeed, but solitary and poorly cultivated, constantly broadening into a lake, or overspreading its banks with swamps. It drains the country between the Transylvanian mountains and the Balkans, forming the northern boundary of Bulgaria as far as Silistria; and from here it turns northward, skirting the Dobruja, and flows between marshy banks to Galatz, receiving on the way the Jalomitzza and the Sereth. From Galatz it flows E., and, after being joined by the Pruth from the N., SE. to the Black Sea. The delta, which begins 5 miles W. of Tildja, is a vast wilderness, covering an area of 1000 sq. m., and resembling an immense green sea of rushes; it is cut up by numerous channels and lakes, and is the haunt of sea-birds, wolves, and buffaloes. The farthest mouths are 60 miles apart. Two-thirds of the Danube's volume passes through the Kilia, but this arm forms a secondary delta near its outlet, and the southern or St George branch also forms two channels; it is consequently by the middle or Sulina mouth that ships enter, although it discharges only of the river's waters. The improvements (1858-95) of this mouth have increased the depth over the bar from 7 or 11 feet (according to the season) to a minimum depth of 23 feet. And cuttings (1890-94) have shortened the course of the Sulina branch by six nautical miles. To defend Vienna against risk of inundation, the course of the Danube skirting it was, in 1868-81, diverted into an artificial channel, and the bed has since been improved, above and below the city, from the mouth of the Isper to that of the March. Similar works have been carried out near Pesth, and a new channel, cut at vast expense, to avoid the impediments to navigation at the Iron Gate, was opened in 1893. Until then any effectual improvements of that passage had never advanced beyond the stage of projects, while throughout Hungary and along the lower course inundations had caused great damage. The operations were actually begun by the Hungarian government in 1890. A great steel cantilever railway bridge across the Danube at Tchernavoda in Roumania (with its approaches, 11,800 yards long) was opened in 1895. It partly rests on the island of Balta. The Danube has about 400 tributaries, 100 of them navigable, and the Danube Steam Navigation Company (1830), which has done much to increase the commerce, possesses nearly 200 steamers and over 700 iron tow-boats; other companies also have placed steamers on the upper portions of the river. Nevertheless, owing to the obstructions and to the shifting course of the stream, the tonnage of the Danube trade is inferior to that of the Elbe. The Danube is connected with the Rhine by means of the Ludwigs-Canal (1844), and with the Elbe by means of the Moldau and Mühl, and canals.
THE INTERNATIONAL DANUBE NAVIGATION COMMISSION was constituted in 1856, when at the Peace of Paris the navigation of the river was declared free to all nations, and was composed of delegates of all the great powers, to whom a representative of Roumania has been added since 1878. It was appointed on the express condition that it should dissolve in 1858, but such was its usefulness that it was informally continued till 1866, when the Conference of Paris formally prolonged its powers for five years. In 1871 the Conference of London continued the Commission for twelve years, and in 1883 a second London conference extended its existence for other twenty-one years. It exercises almost sovereign power on the mouths of the Danube, where it has conducted the great engineering works already referred to; it has its own flag, uniform, and revenue, and has raised loans, made laws, and maintained its own small army of police. Its jurisdiction, originally limited to the river between Isaktcha and the sea, was extended at the Congress of Berlin (1878) as far as Galatz, and afterwards to the Iron Gate; but in the last-named portion of the stream its authority is exercised only by delegation to the Riverain Commission of the states on the bank (also constituted in 1856), or on appeal from its decisions.