A drop of rain that falls in the Black Forest can end up in two different seas. Near the village of Immendingen the young Danube leaks into the limestone riverbed, and on some days of the year it disappears completely, only to come out about 12 kilometers south as a spring that feeds the Rhine and the North Sea. What stays in the channel goes the other way: past four capitals, along or across ten countries, and out through a delta into the Black Sea. The Danube has been shaping who ruled what in Europe for two thousand years, and it still decides where the borders run.
Where the Danube Begins
Ask where the Danube starts and you get two answers. The official one is Donaueschingen in the southern Black Forest, where two small streams, the Brigach and the Breg, meet and the river takes the name Donau. There's even a stone-edged spring basin in the palace park that the town presents as the source. The geographer's answer is different: the Breg is the longer of the two headstreams, and it rises higher up in the hills near Furtwangen. The town has a good claim anyway, because the name begins there.
Then comes the strangest stretch of the whole river. Around Immendingen, in the Swabian Alb, the Danube Sinkhole swallows part of the water through cracks in the karst. It reappears at the Aachtopf, Germany's biggest karst spring, and runs on as the Radolfzeller Aach into Lake Constance. On some days of the year the riverbed here is completely empty. It's a rare place where you can stand on the line between two river systems and watch it leak.
The Numbers, and Why the River Matters
The usual figure is 2,850 kilometers, which makes it the second-longest river in Europe after the Volga. Sources don't agree to the last kilometer: one passage of the Danube article gives about 2,730 elsewhere. The river runs through or along the borders of ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine. Four national capitals sit on its banks, Vienna, Bratislava, Budapest and Belgrade, more than on any other river. The basin covers roughly 800,000 square kilometers and reaches into nine more countries. Places along the river are given by river kilometer, counted upstream from the mouth: the fortress at Ram, for example, sits at kilometer 1,075.
The river has always been a road. Under Rome it was the northern frontier for most of its length, the Danubian Limes, and the western segment of that frontier is now a UNESCO World Heritage Site. Later it divided the Habsburgs from the Ottomans, and in 1856 the powers met in Paris to agree on free navigation, the first international regime for the river. Today the Danube Commission (founded in 1948) looks after navigation. Since the Rhine-Main-Danube Canal opened in 1992, a barge can go from Rotterdam on the North Sea to Sulina on the Black Sea, about 3,500 kilometers. The river also drives hydropower and supplies drinking water to around 20 million people along its course.
And the color? The river is not famously blue. The name comes from Johann Strauss II's Blue Danube waltz, composed in 1866 and first performed in 1867, which took its title from a poem byKarl Isidor Beck, so the blue is more poetry than hydrology.
The Upper Danube: Ulm to Vienna
In Germany the river is still modest, and it picks up the Iller at Ulm and then turns toward Bavaria. At Regensburg it reaches its northernmost point, and the medieval center, the biggest of its kind north of the Alps, has been UNESCO-listed since 2006. Further down, Passau is the City of Three Rivers: the Inn joins from the south and the Ilz from the north, and the Danube leaves for Austria as a much bigger river than it arrived.
Then comes the stretch most people picture when they think of a Danube cruise. The Wachau runs 36 kilometers between Melk and Krems, a valley of terraced vineyards, apricot orchards and small towns, inscribed by UNESCO in 2000. At its western end the yellow baroque Melk Abbey sits on a rock above the river. The Benedictines have been there since 1089, and the building you see was put up between 1702 and 1736. The valley is best seen from the water, which is why so many boats stop here.
Vienna is the first of the four capitals. Our guide 5 Days in Vienna covers the city itself, and the Austria country page has the wider picture.
The Middle Danube: Bratislava to Belgrade
Bratislava is the only capital that borders two other countries, Austria and Hungary, and it sits on both banks of the Danube. Hungary comes next. The river swings south in the Danube Bend, a curve around Visegrád with Esztergom, once the Hungarian capital, and Szentendreclose by. After that it divides Budapest in two. Buda and Pest were separate cities until 1873, and the riverbanks with the castle and the Parliament are a World Heritage Site. If you only see one capital on the Danube from the water, make it this one, at night.
Further south the river crosses the flat Pannonian plain and becomes the border between Croatia and Serbia. Along a section of about 140 kilometers the two countries still disagree about where the line runs: Serbia says it follows the middle of the channel, Croatia says it follows the 19th-centuryriverbed. Inside Serbia, Novi Sad sits on a bend where the Danube is only about 350 meters wide, below the Petrovaradin Fortress, nicknamed the Gibraltar of the Danube. Its famous clock has the hands reversed, withthe short hand showing the minutes, said to be so fishermen on the river could read it from far away. The EXIT festival moved to that hill in 2001. Check the festival's site for where it takes place now.
Belgrade is where the Danube meets the Sava. The old citadel of Kalemegdan stands on the ridge right above the confluence, and the city has grown around it. It's one of the best viewpoints on the whole river: two big waters merge at your feet. Read our 5 Days in Belgrade and the Serbia country page before you go, and 5 Days in Budapest and 3 Days in Bratislava for the stretch upstream. Downstream, Smederevo has a huge fortress built between 1427 and 1430 by Despot Đurađ Branković, a medieval Serbian capital for a time.
The Iron Gates: Đerdap
Past Belgrade the river gets wide, and then it does something unexpected: it squeezes through the Carpathians. The Iron Gates (Đerdap in Serbian) form a gorge on the border between Serbia and Romania, about 134 kilometers long in the broad sense. Before the gorge the Danube is around 5 kilometers wide near Ram, and it is wider still at Golubac, where the gorge begins. Then thewalls close in. At the Great Kazan the river narrows to about 150 meters and is up to about 53 meters deep, the narrowest point of the whole route.
The entrance is guarded by Golubac Fortress, a 14th-century stronghold with ten towers that passed between Serbs, Hungarians,Ottomans and Austrians. Further down on the Romanian side, a 55-meter face of the Dacian king Decebalus looks over the water. It was carved between 1994 and 2004, and it's the tallest rock relief in Europe. Nearby, a Roman bridge once crossed the Danube: Apollodorus of Damascus designed it for Trajan, and it was finished in 105. Only the pier stumps remain. Lepenski Vir, on the Serbian bank, is far older: a settlement of hunter-gatherers with trapezoidalhouses and carved stones, occupied thousands of years before the Romans.
In 1972 the gorge was dammed. The Iron Gate I power station is the largest dam on the river, built jointly by Yugoslavia and Romania, and the reservoir drowned the island of Ada Kaleh. The Serbian side of the gorge is Đerdap National Park, and a wider area has been a UNESCO Global Geopark since 2020.
The Lower Danube and the Delta
After the dam the Danube becomes the border between Romania on the north bank and Bulgaria on the south, and the bridges are rare. For decades the only one connecting the two countries was the Danube Bridge between Ruse and Giurgiu, opened in 1954. It's 2,223 meters long, has a road and a railway on two decks, and it was long called the Friendship Bridge. Ruse itself is known as Little Vienna for its 19th-century facades. A second crossing, the New Europe Bridge between Vidin and Calafat, opened only in 2013.
Which bridge is the biggest depends on the word. The Brăila Bridge, a suspension bridge opened on 6 July 2023, is described as the largest bridge over theDanube: it's 1,974 meters long with a main span of 1,120 meters. The Ruse bridge is longer in total length, but its spans are far shorter, so pick whichever record you like.
Then the river splits and the land turns to water. The Danube Delta covers about 4,150 square kilometers, mostly in Romania with a northern part in Ukraine, and it's the largest river delta in the European Union. Three main arms, Chilia, Sulina and Sfântul Gheorghe, spread out into reed beds, lakes and channels. More than 320 bird species have been recorded here in summer, from pelicans to pygmy cormorants. The Romanian part has been a World Heritage Site since 1991, and the whole delta a biosphere reserve since 1998. You get there from Tulcea by boat, and the Sulina arm, the shortest and the one used by ships, ends at a port town of about 4,600 people. Our 5 Days in Bucharest and the Romania country page are the natural start.
Battles on and Around the Danube
A river that marks a frontier collects battles. The ones below all happened on or near the Danube.
- 101 to 106: Trajan's Dacian wars. The Romans cross the Danube near the Iron Gates, and after the victory over Decebalus they create the province of Dacia.
- 1456: John Hunyadi's relief force destroys an Ottoman flotilla on the Danube on 14 July, and the siege of Belgrade ends on 22 July with an Ottoman retreat.
- 1526: At Mohács on 29 August the Ottomans wipe out the Hungarian royal army, and King Louis II dies.
- 1683: The Ottomans, after two months of siege, are beaten at Vienna on 12 September, and they never threaten the city again.
- 1704: Blenheim, in Bavaria near the Danube, secures Vienna from a Franco-Bavarian army.
- 1716: Prince Eugene of Savoy beats the Ottoman army at Petrovaradin on 5 August, at the fortress above today's Novi Sad.
- 1809: At Aspern-Essling in May, Napoleon is forced back across the Danube, his first major defeat. In July he wins at Wagram, one of the largest battles in Europe up to that time.
- 1914: Austria-Hungary declares war on Serbia on 28 July, and its river monitors shell Belgrade the next day.
- 1944 to 1945: The siege of Budapest lasts from 26 December to 13 February, and all seven Danube bridges in the city are destroyed.
Travelling the Danube
The easiest way is a river cruise. Routes and seasons differ a lot between operators, so check theirs before you plan. If you prefer pedals, EuroVelo 6 follows the Danube Cycleway from Tuttlingen in Germany to the Black Sea, as part of a 3,653-kilometer route that starts on the Atlantic coast of France. You can also hop between cities by train and see the river in pieces.
If you want to pick one stretch, pick Belgrade and the Iron Gates for drama, Budapest for the best city on the water, and the Wachau for the classic postcard. Start with our guides to Vienna, Budapest, Belgrade and Freiburg, which is a short trip from the Black Forest source. And if you'd like to test how wellyou know the rivers and capitals of Europe, try our geography games.
