The Yellow River stretches 5,464 kilometers from the glaciers of the Bayan Har Mountains on the Qinghai-Tibet Plateau to its delta on the Bohai Sea in Shandong Province. It is China's second-longest river after the Yangtze, and Chinese civilization grew along its banks in the North China Plain. The earliest Chinese dynasties -- the Xia, Shang, and Zhou -- all rose in its watershed. For this reason, the Chinese call it the Mother River. But the Mother River has also been history's deadliest. Over the past two millennia, the Yellow River has changed its main course at least five times, sometimes emptying into the Yellow Sea hundreds of kilometers south of its current mouth. Each shift brought flooding on a scale difficult to comprehend: the 1887 flood killed between 900,000 and two million people, and the 1931 flood may have killed as many as four million.

The river's destructive power comes from an unusual geological relationship. In its middle course, the Yellow River cuts through the Loess Plateau, where wind-deposited silt has accumulated to depths of 100 meters or more over millions of years. The river erodes this soft material with extraordinary efficiency, loading itself with sediment. When the river reaches the flat North China Plain, it slows down and drops that sediment on its own bed. Over centuries, the riverbed rises above the surrounding floodplain, held in place only by levees that communities have built higher and higher since the 6th century BC. In some stretches near Kaifeng, the riverbed sits ten meters above the surrounding farmland. When the levees break -- and historically they have broken with devastating regularity -- the water pours across the plain with nothing to stop it.

In 1938, during the Second Sino-Japanese War, Chinese Nationalist forces deliberately breached the Yellow River's dikes at Huayuankou to halt the advancing Japanese army. The flood succeeded in slowing the invasion, but it also killed an estimated 500,000 to 900,000 Chinese civilians and displaced millions more. The river did not return to its previous course for nine years. This was not the first time the river was used as a weapon -- in 1128, Song dynasty general Du Chong breached the southern dike to slow advancing Jurchen armies, shifting the Yellow River's mouth from the Bohai Sea to the Yellow Sea, where it would remain for over seven centuries.

Modern engineering has transformed but not conquered the Yellow River. Ten large dams now regulate its flow, and the river has not experienced a major flood since 1945. But new problems have emerged. In the 1990s, the river repeatedly ran dry before reaching the sea -- in 1997, the lower Yellow River had no flow for 226 days. Pollution and water diversion have devastated the river's ecology; a 2007 report found that one-third of its fish species had been killed off. The Yellow River delta itself is shrinking, losing roughly 7.6 square kilometers annually. Tourist attractions along its course -- the Kanbula Geopark in Qinghai, the ancient Bingling Grottoes at Liujiaxia Dam, the Yellow River Stone Forest in Gansu, and the thundering Hukou Waterfall in Shanxi -- draw visitors to a river that remains as central to China's identity as it is to its geography.