Workers gather and bundle reeds in the Danube River Delta, which harbors the largest stands of reeds on Earth. The plants play a crucial role in the delta's ecology, filtering the water and providing habitat for a wide range of wildlife.

Sometimes it's hard to tell where a river begins. It's doubly difficult with the Danube, born of two rival sources, both trickling down the slopes of Germany's Black Forest. But such ambiguity seems fitting, for the Danube is hard to pin down. Its name conjures graceful couples waltzing the night away in 19th-century Vienna, and for the first third of its nearly 1,800-mile (2,900-kilometer) journey to the Black Sea it plies an idyllic course beside tidy villages and storybook castles. But there is another Danube River, mainly in eastern Europe, one damaged by pollution and war. A NATO bombing campaign against Yugoslavia in 1999 destroyed industrial facilities, releasing contaminants into the water, and blew up three main bridges, which halted or diverted as much as a billion dollars in business.

But the Danube had a dual personality even before the bombing. The river is a playground for swimming and boating; it's also very much a working river of fishermen, barges, power plants, and shipyards. The working river is part of a busy transportation corridor connecting the North Sea with the Black Sea through the Main-Danube Canal, but it also feeds wetlands and valuable wildlife habitat. Today people along the river are striving to reconcile these competing demands and are beginning to undo years of environmental and war damage.

When I sought the river's source, an April snow shower greeted me on the farm of Hansjörg and Beate Heinzmann in the mountains of the Black Forest. Tapping the headwaters of the Brigach River, Frau Heinzmann held a blue-flowered pitcher below an opening in a stone slab near the farmhouse and then filled my glass. Downriver I would taste the wines of Austria. Here I swirled this cold, fresh vintage in my mouth. With just a hint of mineral tang, it was absolutely refreshing.

The Brigach has become a shrine for people from the ten countries through which the Danube flows. "We get a lot of visitors, mostly drop-ins," said Frau Heinzmann. When I stopped by at 11 a.m., 65 people had already visited that day. Drawing on the flow of travelers, the Heinzmanns have built a bed-and-breakfast on their farm.

A rival source lies near the city of Furtwangen, where waters of the Breg River flow through another stone marker next to a resort hotel. That stream has a stronger claim since it begins farther from the Danube's mouth. Underground streams from the Breg and Brigach meet in an ornate stone and concrete pool next to Fürstenberg Palace in the city of Donaueschingen. Only there does the river—Europe's second longest, after the Volga—take the name Danube.

I began following its course at Ulm, the first navigable point on the river, here patrolled by graceful swans. The world's tallest cathedral spire crowns Ulm's Gothic structure. From the top you can survey what was the medieval duchy of Swabia, one of a procession of powers—Celts, Romans, the Holy Roman Empire, Ottoman Turks, the Habsburgs, the Austro-Hungarian Empire, Nazi Germany—that has held sway within the long Danube Basin. An outpost of the Roman Empire remains about 110 miles (177 kilometers) downriver at Regensburg, where walls of a fortress built in a.d. 179 still stand.

Leaving Germany, the Danube flows eastward into Austria, whose Wachau Valley is fertile ground for both piety and wine. Terraces of vineyards climb slopes as if to seek benediction from the great medieval monasteries that command the hilltops at Melk and Krems. The river passes castle ruins and apricot orchards that spangle the banks with snowy blossoms.

Wachau's scenery is captivating, but no place is as identified with the Danube's charm as nearby Vienna. At the Café Landtmann, where Sigmund Freud took his morning coffee, I savored a strong brew while sifting through racks of out-of-town newspapers. And in the shabbily genteel Liechtenstein Palace I took in a performance of the "Blue Danube" waltz, the melody that evokes Vienna the world over.

Vienna was a hub of the Habsburg empire, and in the 1860s, after Austria's defeat by the Prussians, Johann Strauss wrote the "Blue Danube" to cheer up his fellow Austrians. His music was "an evocative bow," wrote modern critic Harold C. Schonberg, to "a Vienna of young hussars and beautiful ladies, a Vienna of sentimentality and charm, a pretty-pretty and never-never Vienna of dance and romance."

Downriver from Vienna the Danube cleaves Hungary's capital of Budapest, with Pest on the left bank, commanded by the stately parliament building, and Buda on the right, crowned by Castle Hill, where King Béla IV built his stronghold against Mongol invaders in 1255. Between the banks sits Margaret Island, named for Béla's daughter, where I strolled paths flanked by gazebos, playgrounds, band shells, and young couples entwined on shaded benches like living statues.

Like other central and east European countries, Hungary shed its Soviet-dominated government more than a decade ago. But it's still plagued by an international dispute rooted in the communist era. In 1977 Hungary and Czechoslovakia agreed to build dams—one by each country—on the Danube where it winds along their border. Czechoslovakia (which split into Slovakia and the Czech Republic in 1993) built a dam and a shipping canal that diverted water from the river, adversely affecting wetlands and water quality. But after Hungary rejected communism, concern about costs, water supplies, and environmental damage led it to halt plans for its dam. The result was a massive joint public works project only partly finished—symbolic of the conflicts that arise as a new post-communist sensibility struggles to be born.

South of Budapest I found the river in a happier state. At Duna-Dráva National Park I rode with park ranger Barnabás Felső in a small boat to one of the many islands in the park's flooded landscape. He pulled up to a fenced plot that he's turned into a kind of open-air retreat and toolshed. From the river he hauled a net that yielded four vigorously flopping fish he called golden ide, which were soon sizzling in a cauldron over a smoky fire.