Physiography

The Danube’s vast drainage of some 315,000 square miles (817,000 square km) includes a variety of natural conditions that affect the origins and the regimes of its watercourses. They favour the formation of a branching, dense, deepwater river network that includes some 300 tributaries, more than 30 of which are navigable. The river basin expands unevenly along its length. It covers about 18,000 square miles (47,000 square km) at the Innconfluence, 81,000 square miles (210,000 square km) after joining with the Drava, and 228,000 square miles (590,000 square km) below theconfluences of its mostaffluent tributaries, theSava and theTisza. In the lower course the basin’s rate of growth decreases. More than half of the entire Danube basin is drained by its right-bank tributaries, which collect their waters from the Alps and other mountain areas and contribute up to two-thirds of the total river runoff or outfall.

Three sections are discernible in the river’s basin. The upper course stretches from its source to the gorge called theHungarian Gates, in the Austrian Alps and the WesternCarpathian Mountains. The middle course runs from the Hungarian Gates Gorge to theIron Gate in the Southern RomanianCarpathians. The lower course flows from the Iron Gate to the deltalike estuary at theBlack Sea.

The upper Danube springs as two small streams—theBreg andBrigach—from the eastern slopes of theBlack Forest mountains of Germany, which partially consist oflimestone. From Donaueschingen, where the headstreams unite, the Danube flows northeastward in a narrow, rocky bed. To the north rise the wooded slopes of the Swabian and the Franconian mountains. BetweenIngolstadt andRegensburg the river forms a scenic canyonlike valley. To the south of the river course stretches the large BavarianPlateau, covered with thick layers of river deposits from the numerous Alpine tributaries. The bank is low and uniform, composed mainly of fields, peat, and marshland.

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AtRegensburg the Danube reaches its northernmost point, from which it veers south and crosses wide, fertile, and level country. Shortly before it reaches Passau on the Austrian border, the river narrows and its bottom abounds with reefs and shoals. The Danube then flows through Austrian territory, where it cuts into the slopes of theBohemian Forest and forms a narrow valley. In order to improve navigation, dams and protecting dikes have been built nearPassau,Linz, and Ardagger. The upper Danube, some 600 miles (965 km) long, has a considerable averageinclination of the riverbed (0.93 percent) and a rapid current of two to five miles per hour. Depths vary from 3 to 26 feet (1 to 8 metres). The Danube swells substantially at Passau where theInn River, its largest upstream tributary, carries more water than the main river. Other major tributaries in the upper Danube course include the Iller, Lech,Isar, Traun, Enns, and Morava rivers.

In its middle course the Danube looks more like a flatland river, with low banks and a bed that reaches a width of more than one mile. Only in two sectors—at Visegrád (Hungary) and at the Iron Gate—does the river flow through narrow canyonlike gorges. The basin of the middle Danube exhibits two main features: the flatland of theLittle AlfoldandGreat Alfold plains and the low peaks of the Western Carpathians and Transdanubian Mountains.

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The Danube enters the Little Alfold plain immediately after emerging from the Hungarian Gates Gorge nearBratislava, Slovakia. There the river stream slows down abruptly and loses its transporting capacity, so that enormous quantities of gravel and sand settle on the bottom. A principal result of thisdeposition has been the formation of two islands, one on the Slovak side of the river and the other on the Hungarian side, which combined have an area of about 730 square miles (1,900 square km) that support some 190,000 inhabitants in more than 100 settlements. The silting hampers navigation and occasionally divides the river into two or more channels. East ofKomárno the Danube enters theVisegrád Gorge, squeezed between the foothills of the Western Carpathian and the Hungarian Transdanubian Mountains. The steep right bank is crowned with fortresses, castles, and cathedrals of the HungarianÁrpád dynasty of the 10th to 15th century.

The Danube then flows past Budapest, and across the vast Great Alfold plain,traversing Croatia, Serbia, and Romania until it reaches the Iron Gate gorge. The riverbed is shallow and marshy, and low terraces stretch along both banks. River accumulation has built a large number of islands, includingCsepel Island near Budapest. In this long stretch the river takes on the waters of its major tributaries—the Drava, the Tisza, and the Sava—which create substantial changes in the river’s regime. The average runoff increases from about 83,000 cubic feet (2,400 cubic metres) per second north of Budapest to 200,000 cubic feet (5,600 cubic metres) at the Iron Gate. The river valley looks mostimposing there, and the river’s depth and current velocity fluctuate widely. The rapids and reefs of the Iron Gate once made the river unnavigable until a lateral navigation channel and a parallel railway allowed rivercraft to be towed upstream against the strong current.

Beyond the Iron Gate the lower Danube flows across a wide plain; the river becomes shallower and broader, and its current slows down. To the right, above steep banks, stretches the tableland of theDanubian Plain of Bulgaria. To the left lies the lowRomanian Plain, which is separated from the main stream by a strip of lakes and swamps. The tributaries in this section are comparatively small and account for only a modest increase in the totalrunoff. They include theOlt, theSiret, and thePrut. The river is again obstructed by a number of islands. Just south of Cernavodă, the Danube heads northward until it reaches Galați, where it veers abruptly eastward, its left bank briefly traversing Moldovan territory. NearTulcea, Romania, some 50 miles (80 km) from the sea, the river begins to spread out into its delta.

The river splits into three channels: theChilia, which carries 63 percent of the total runoff; theSulina, which accounts for 16 percent; and theSfântu Gheorghe (St. George), which carries the remainder. Navigation is possible only by way of the Sulina Channel, which has been straightened and dredged along its 39-mile (63-km) length. Between the channels, a maze of smaller creeks and lakes are separated by oblong strips of land calledgrinduri. Mostgrinduri are arable andcultivated, and some are overgrown with tall oak forests. A large quantity of reeds that grow in the shallow-water tracts are used in the manufacture of paper and textile fibres. The Danubedelta covers an area of some 1,660 square miles (4,300 square km) and is a comparatively young formation. About 6,500 years ago the delta site was a shallow cove of the Black Sea coast, but it was gradually filled by river-borne silt; the delta continues to grow seaward at the rate of 80 to 100 feet (24 to 30 metres) annually.