Danube (anc. Danubius, or, in its lower course, Ister; Ger. Donau), the largest river of Germany, and, next to the Volga, of Europe. It is formed by the confluence of two streams, the Brege and Brigach, rising in the grand duchy of Baden, on the S. E. slope of the Black Forest, the former, which is considered the principal source of the river, in lat. 48° 6' N., lon. 8° 9' E., about 24 m. from the Rhine, at an elevation of about 2,850 ft. above the level of the Black sea. In an air line the distance from the sources to the mouth of the Danube is nearly 1,020 m., while the length of its course is 1,820 m. The river system of the Danube and all its tributaries drains an area of 300,000 sq. m. In its course it traverses nearly 22° of longitude and 5 1/2° of latitude. The elevation of its surface above the level of the sea at Ulm, the head of steam navigation, is about 1,500 ft., at Ratisbon 1,100, at Vienna 500, at Presburg 425, at Buda 350, and at Moldova 200. Three principal divisions of the river basin are indicated by the character of the adjacent country, and the river itself: the upper course, terminating at Passau; the middle, at Kladova; and the lower, at the mouth.

In its upper course the Danube, flowing in an easterly direction, skirts the southern base of the sterile table land of the Rough Alps (Rauhe Alp), the rapidity of its current being 5 1/2 ft. a second, or about 3 1/2 m. an hour. Its breadth having increased to 250 ft., and its current slackened, it becomes navigable at Ulm. There, sweeping to the N. E. through the fertile Bavarian plain, it forms a curve, of which the northern apex is Ratisbon, and the eastern base Passau. On the N. side the Ludwigs-canal connects it with the Main and Rhine rivers. Near Passau the Bavarian forest on one side, and the northern ranges of the Noric Alps on the other, approach the Danube, narrowing its bed in some places to less than 1,000 ft., while in others it expands to a breadth of 5,000 ft. From Passau to Linz the fall is 2 1/4 ft. in a mile; from there to Vienna only 1 3/4 ft. In this portion of its course the scenery of the Danube fairly rivals that of the Rhine, and even excels it in sombre grandeur. Nearer Vienna the mountains recede, and the river enters a large plain, which, being but scantily protected by dikes, is subject to terrible inundations.

The measures heretofore adopted for preventing these inundations having been found wholly inadequate, the Austrian government in 1864 appointed a committee, consisting of representatives of the monarchy and of the crown-land of Lower Austria and the city of Vienna, to regulate the course of the Danube at and near Vienna. The plan of the committee, which involves the leading of the river into a new channel, was approved in 1868. The cost of the work is estimated at 24,600,000 florins, of which one third is to be paid by the monarchy, one third by Lower Austria, and one third by Vienna. The principal portions of the work are to be completed during 1874.

Passing again between two mountain ranges, the Leitha on the southern and the lesser Carpathians on the northern bank, the Danube emerges into the fertile region of western Hungary. There, spreading out in several branches, it forms a great number of islands, among which the Great Schutt (50 m. long) and the Little Schutt (27 m. long) are the largest. Through a defile, formed by the Nograd branch of the Carpathians and the Bakony Forest, the Danube enters the great Hungarian plain, turns abruptly S. near Waitzen, and slowly winds through vast level bottom lands and marshes, until it meets the Sirmian range, and, having received the waters of the Drave, is again deflected toward the southeast. It then skirts the plain on the south till near Moldova, where it passes through the Transylvanian granite hills and the Servian limestone range. This pass (Kli-sura), 80 m. long, offers the greatest obstacles to the navigation of the Danube. Narrowed down to less than one half its former breadth, the river forms in seven places between Alibeg and Kladova rapids and whirlpools, of which those in the so-called Iron Gate, below Old Orsova, are the most violent.

There it pours through a defile 7,500 ft. long and 650 wide, with a fall of 16 ft., and a rapidity of 10 to 15 ft. a second, over a number of reefs and ledges. After having been a terror to navigators for centuries, the Iron Gate has at last been rendered navigable for steamers, a channel having been cut through the ledge, by which vessels ply from Vienna to Galatz without a portage. In ancient times this portion of the river course was avoided by a canal, of which some vestiges still remain. Near Kladova the Danube enters the Bulgaro-Wallachian plains. From Tcher-netz to below Widin it runs nearly S., then turns E. Slowly rolling its muddy waters round the extreme spurs of the Balkan, and forming numerous islands, it reaches a point only 32 m. from the sea, where it suddenly bends N. and flows upward of 100 m. to the junction with the Sereth; thence again eastward; at last, joined by the Pruth and divided into several branches, which sluggishly wind through a low and dreary alluvial country (the delta of the Danube), it empties into the Black sea by three principal channels, the Kilia, Sulina, and St. George, and four lesser ones. - The most important tributaries of the Danube are, on the right or southern bank, the Iller, Lech, Isar, Inn, Traun, Enns, Leitha, Raab, Sarviz, Drave, Save, and Morava; on the left bank, the Brenz, Warnitz, Altmuhl, Naab, Regen, Itz, March, Waag, 'Neutra, Gran, Eipel, Theiss, Temes, Aluta, Arjish, Yalomitza, Sereth, and Pruth. The principal towns on its banks are: in Wtirtemberg, Ulm; in Bavaria, Ratisbon and Pas-sau; in Austria proper, Linz and Vienna; in Hungary, Presburg, Comorn, Gran, Buda, and Pesth; in the Military Frontier, Peterwardein and Orsova; in Servia, Belgrade; in Bulgaria, Widin, Nicopolis, Bustchuk, Silistria, and Hirsova; in Roumania, Giurgevo, Braila, and Galatz. - The Danube has through all history been of great political importance.

For the Huns, the Avars, Bulgarians, Magyars, and Tartars, the Danube valley was the scene of efforts to subdue the Occident. While the progress of the barbarians was somewhat checked by the other great European rivers flowing N. and S., the Danube served as a highway to the west. The western nations having at last established their supremacy, the human current was reversed for the conquest of the Orient by the crusaders; and during sueceeding centuries the countries bordering on the Danube were frequently the theatre of conflict between the Christians and the Turks. The Germans occupy the entire upper basin, and portions of the middle and lower; the Slavs parts of both banks of the middle course of the river; the Magyars the central portion of the valley, and the Roumanians the lower regions. - The commercial use of the Danube has scarcely begun to be developed. The rapidity of the current in its upper course, the reefs, rapids, whirlpools, and sudden changes of the channel and banks, its shallowness where it passes through the Hungarian plains, and its numerous windings, offered so many impediments to navigation, that up to a comparatively recent period it was limited to the scantiest intercourse between the provinces immediately adjoining the river.

The application of steam in 1830 inaugurated a new era in the history of the Danube. Then the governments, becoming aware of the importance of the river, adopted a system of improvements. Eeefs were removed, flats deepened by narrowing the channel, and canals and cut-offs constructed. By the treaty of Paris (1856) the entire freedom of the navigation from tolls and dues was stipulated for; and in pursuance of this, the governments of the states through which the river flows agreed upon a convention (Nov. 7, 1857), by which vessels of all nations were allowed to ascend the Danube from its mouth to any point above, while the navigation between different points on the river was reserved to the subjects of the riparian states. The provisions of the treaty of Paris were partially modified by the treaty of March 13, 1871, which authorizes the riparian states, in case of their agreement as to the removal of the remaining obstructions at the Iron Gate, to levy a provisional tax on all commercial vessels availing themselves of its advantages, until the expenditure shall have been repaid. The principal drawback to the importance of the Danube as a channel of commerce is the shallowness of its mouths.

The three outlets enclosing the delta (or rather the flat islands Tchetal, Leti, and Moishe, the highest elevation of which is not more than 6 or 7 ft. above the level of the sea), the Kilia Boghasi, Sulina Bo-ghasi, and Kediskeh Boghasi (St. George channel), have a length of 72, 53, and 55 m. respectively, and according to the report of E. De-jardins, submitted to the prince of Roumania in 1867, convey the waters of the Danube in the respective proportions of 17/27,2/27, and 8/27 of the volume of the river. Formerly the St. George's channel was used almost exclusively; but having been choked with sand by the simultaneous occurrence of a strong freshet in the river and a violent gale from the sea, it was abandoned, and the Sulina channel resorted to. The Turkish government took good care to keep this channel open; but when, by the treaties of 1812 and 1829, the mouths of the Danube passed under the control of Russia, all efforts in that direction were abandoned, with the intention, it was said, of diverting the Danubian trade into the Kilia branch for the advantage of the harbor of Ismail. The bar of the Sulina continues 1,000 yards outside of the mouth; it has a width of 2 to 3 m., and a depth of water varying from 10 to 14 ft.

The energetic efforts of the Austrian government immediately after the retreat of the Russians (1854) at last succeeded so far as to obtain an available average depth of 10 ft., while under Russian rule the depth had been reduced to 7 1/2 ft. By the treaty of Paris an international committee was appointed for the regulation of the mouths of the Danube. The majority of this committee concluded that it would be feasible to suppress or cut off two of the principal outlets, and, by leading their waters into the third, increase the volume and power of the current, sufficiently to sweep away the mud and sand banks. A special committee, appointed by Austria, reported in 1857 that the St. George channel, if restored to its former condition, would in all respects offer the shortest and safest outlet, and that the cost of this improvement would not exceed 3,700,000 florins, nor the cost of keeping it in repair 65,000 florins per annum. The Danube and its principal tributaries (the Inn, Drave, Theiss, and Save) are navigated by steam vessels for an aggregate length of 2,400 m. The Bavarian Danube steam navigation company was established in 1838, and in 1862 was merged in the Austrian company, which was organized in 1830. In 1870 the latter company employed 155 steamers and 547 transports.

The voyage from Vienna to Constantinople is now performed in seven days.

The Iron Gate of the Danube.

The Iron Gate of the Danube.