Danube

The Danube is a long, long river in Europe.

It starts as two little streams in a forest. It flows east past big cities, castles and farms. It ends in a huge marsh by the sea, full of birds.

The Danube is the second-longest river in Europe; only the Volga is longer. It begins in the Black Forest mountains of Germany and flows about 2,850 kilometers east and south to the Black Sea.

On the way, it passes through or along 10 countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova and Ukraine. Four capital cities stand on its banks: Vienna, Bratislava, Budapest and Belgrade. Boats carry goods along it, dams make electricity from it, and farmers water their fields with it.

Britannica calls the Danube the second-longest river in Europe after the Volga, rising in the Black Forest of western Germany and flowing some 2,850 km to the Black Sea through Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova and Ukraine. Its drainage basin of about 817,000 km² is fed by some 300 tributaries, more than 30 of them navigable; the right-bank tributaries, from the Alps and other mountains, drain more than half the basin and supply up to two-thirds of the flow.

The river divides into three courses: the upper Danube from the source to the Hungarian Gates gorge near Bratislava; the middle Danube across the Hungarian plains to the Iron Gate gorge between Serbia and Romania; and the lower Danube from the Iron Gate to the Black Sea. The capitals Vienna, Bratislava, Budapest and Belgrade have grown up on its banks, and its countries use it for freight, hydroelectricity, water supply, irrigation and fishing.

The Danube starts as two small streams in the Black Forest.

They join to make one river. Many other rivers flow in and make it bigger. At the sea, the river splits into three arms and makes a giant marsh.

The Danube starts as two small streams, the Breg and the Brigach, on the eastern slopes of Germany's Black Forest. They join at the town of Donaueschingen. In its upper course the young river runs fast through a narrow, rocky bed. At Passau, the Inn River joins it and brings more water than the Danube itself carries there.

In Hungary the river slows down across wide, flat plains and becomes more than a mile wide. Big rivers such as the Drava, the Tisza and the Sava join it. Then it squeezes through the Iron Gate gorge. Near the Black Sea it splits into three channels and builds a delta, flat land made of mud the river carries. About 6,500 years ago that place was a shallow bay of the sea.

The Breg and Brigach rise on the eastern, partly limestone slopes of the Black Forest and unite at Donaueschingen. The upper Danube, some 965 km long, has what Britannica calls a considerable average slope (0.93 percent) with currents of two to five miles an hour; at Passau the Inn, its largest upstream tributary, carries more water than the main stream. Below the Hungarian Gates the current slows abruptly and drops gravel and sand, building two large islands that support some 190,000 people in more than 100 settlements.

Across the Great Alfold plain the Drava, Tisza and Sava raise the average flow from about 2,400 m³/s north of Budapest to 5,600 m³/s at the Iron Gate. Below Tulcea, about 80 km from the sea, the river splits into the Chilia (63 percent of the flow), Sulina (16 percent) and Sfântu Gheorghe channels. The delta, about 4,300 km², is young: some 6,500 years ago it was a shallow cove of the Black Sea, gradually filled by river silt, and it still grows seaward 24 to 30 m a year. The river's sediment is nearly all quartz grains.

Lots of birds live in the Danube's marshes.

Fish swim in its lakes and channels. Big fish called sturgeons swim up the river from the sea. Dams now block their way.

UNESCO protects the Danube Delta in Romania. It calls it the largest and best preserved of Europe's deltas, with more than 300 kinds of birds and 45 kinds of freshwater fish in its lakes and marshes. Tall reeds grow in its shallow water, and between the channels lie strips of land with farms and oak forests.

Sturgeons are big fish that swim from the Black Sea up the Danube to lay their eggs. Two big dams at the Iron Gates, the first finished in 1972, now stop them about 863 kilometers up the river from the delta, cutting them off from old spawning places in the middle Danube. WWF wants the dams made passable, and scientists are designing fish passes to help the sturgeons get by.

The Danube Delta was inscribed by UNESCO in 1991 (312,440 ha). Its lakes and marshes hold more than 300 bird species and 45 freshwater fish species. Britannica describes the grinduri, oblong strips of land between the channels, many farmed and some covered in tall oak forest, and reed beds whose reeds are harvested for paper and textile fibers.

Danube sturgeons migrate upriver to spawn, which makes them especially vulnerable to dams. WWF notes that the two Iron Gates dams, at river kilometers 942 and 863 above the delta, below the gorge between Romania and Serbia, confine migratory sturgeons to the lowest 863 km of the river and cut off important spawning sites in the middle Danube; Britannica dates the first Romanian-Yugoslav dam and power plant to 1972. Serbia's Institute for Multidisciplinary Research calls the dams impassable barriers and reports that the WePass 2 project has prepared preliminary designs and cost estimates for fish passages there, aiming to restore migration between the Black Sea and the upper basin. Britannica adds that pollution from industry, sewage and farm runoff has diminished the river's once-rich fishing grounds.