Danube River

At about 2,850 kilometres (1,770 miles), the Danube is Europe's second-longest river after the Volga. Its headstreams, the Breg and Brigach, rise in Germany's Black Forest and join at Donaueschingen. From there the river travels generally east and southeast across central and southeastern Europe before entering the Black Sea. The main channel passes through ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova and Ukraine.

A river basin includes the tributaries, lakes and land that drain toward a river, not just its main channel. The Danube basin covers more than 800,000 square kilometres—about a tenth of continental Europe—and extends into the territories of 19 countries. The International Commission for the Protection of the Danube River (ICPDR) estimates that more than 80 million people live in the basin. Vienna, Bratislava, Budapest and Belgrade are national capitals directly on the Danube.

The Danube's surroundings change along its course. Its upper reaches descend from the Black Forest through upland valleys and Alpine-influenced landscapes. Farther downstream it crosses broad plains and alternates between open lowlands and narrow gorges, including the Iron Gate between Serbia and Romania. Major tributaries such as the Inn, Drava, Sava and Tisza contribute water from large parts of the basin.

Near Tulcea in Romania, the river branches into a delta before reaching the Black Sea. Its three principal distributaries are the Chilia, Sulina and Sfântu Gheorghe (St George) channels. Their shifting channels, lakes, marshes and reedbeds are shaped by the movement of river water and sediment.

The river has long served as a route for movement and exchange between central and southeastern Europe. Today it supports freight transport, hydropower, water supplies, irrigation, industry and fishing. Its cities include important ports, while canals extend its navigable connections: the Main–Danube Canal, completed in 1992, links the Danube to the Rhine and onward to the North Sea.

Navigation is governed through international cooperation. The Danube Commission was established under the 1948 Convention regarding the Regime of Navigation on the Danube. Its work focuses on free navigation for commercial vessels, common rules and navigation safety. This remit is distinct from basin-wide environmental coordination, which is led by the ICPDR and its member countries.

At the river's mouth, the Danube Delta is a large network of channels, lakes, marshes and reedbeds. The UNESCO World Heritage Centre describes it as Europe's largest and best-preserved delta and records its rich birdlife and freshwater fish. The wider Danube Delta Biosphere Reserve is shared by Romania and Ukraine and includes important breeding and stopover habitats for birds. Fishing, reed harvesting, livestock raising, agriculture and tourism are among the livelihoods found in the reserve.

The delta illustrates how river systems connect distant places: water, sediment, nutrients and human impacts from upstream can affect the conditions where the river meets the sea.

Because the basin crosses national borders, decisions about water quality, floods and habitats require cooperation among countries. The ICPDR coordinates basin-wide action under the Danube River Protection Convention, signed in 1994. Its 2021 management-plan update identified five major basin-wide water-management issues: pollution by organic substances, nutrients and hazardous substances; hydromorphological alterations such as changes to river form and flow; and climate-change effects, including drought, water scarcity and extreme hydrological events. These are pressures assessed across the basin, whose rivers, wetlands and communities are interconnected.