Nearly 4,000 years ago, a landslide sent boulders and sediment tumbling into a valley of the Yellow River. The carnage created a massive earthen dam some 660 feet (200 meters) tall, cutting off the river for months.

When that dam finally burst and the river broke free, a massive flood raged across the countryside—and potentially altered the course of Chinese history.

That’s the story told by sediments and archaeological remains described Thursday in a provocative new study published in Science. If correct, the geologic evidence provides a kernel of truth to one of the country’s most important legends: a great flood that paved the way for the Xia, China’s semi-mythical first dynasty.

“Its importance is just like the story of Noah’s flood in the Western world,” says study leader Qinglong Wu of China’s Peking University.

According to the legend, ancient China held a vast watery landscape that took decades to make livable, largely through the efforts of a hero named Yu. For his work, he was rewarded with political power, ultimately founding the Xia dynasty.

There’s considerable debate, however, over whether the Xia actually existed. The main evidence comes from stories written down centuries after their rule, and no archaeologically recovered writings have been concretely tied to the Xia.

If the newly discovered flood is the great flood of legend, it offers tantalizing evidence for the tale. For starters, the flood dates to 1920 B.C., a period that coincides with a critical time in Chinese history: the beginning of the Bronze Age and the start of the Erlitou culture, which some archaeologists associate with the Xia.

“If the great flood really happened, then perhaps it is also likely that the Xia dynasty really existed too. The two are directly tied to each other,” says study co-author David Cohen of National Taiwan University. (Read about a National Geographic photographer's epic 3,000-mile journey across China.)

Trickle of Evidence

The discovery marks the end of a nine-year journey for Wu, who first noticed evidence for the flood in 2007 after examining the Jishi Gorge, an upstream river valley for the Yellow River.

Fieldwork and Google Earth photos revealed yellowish deposits in the gorge that looked like they had been left by a lake, which meant that at some point, the river must have been blocked.

Wu then examined the archaeological site of Lajia, a collection of cave dwellings some 16 miles (25 kilometers) downstream that was destroyed by an earthquake. Radiocarbon dating of human bones at this site date the destruction to about 3,900 years ago.

The site was caked with a distinctive black sand unlike the sediments nearby—and Wu noted that the sediments must have washed into Lajia less than a year after the earthquake. Analysis revealed that the sediments came from upstream, right around the Jishi Gorge.

View Images Jishi Gorge, upstream of what was once the landslide dam. The yellowish deposits above the modern water level are from the ancient lake that the dam temporarily created. Photograph by Wu Qinglong

Soon after, Wu found his smoking gun: remnants of an earthen dam knocked into the Jishi Gorge by a landslide. He published his discovery in 2009—but only afterward did he realize that the natural dam had been much bigger. Reexaminations of the area revealed additional dam remnants, suggesting that it was a behemoth about half a mile (800 meters) wide, three-quarters of a mile (1,300 meters) long, and 660 feet (200 meters) tall.

“That’s as big as the Hoover Dam or the Three Gorges Dam,” says Purdue University geologist Darryl Granger, one of the study’s co-authors. “Imagine a dam like that failing.”

Based on the team’s revised calculations, the ancient flood released nine months’ worth of river water in a matter of hours. At the flood’s peak, 160 Olympic swimming pools’ worth of water coursed down the river every second.

At Lajia, floodwaters reached heights of up to 131 feet (40 meters) above the river’s normal banks. The deluge may have even changed the Yellow River’s course in the lowlands hundreds of miles away, triggering watery conditions that could have lasted for years. (Read more about China's plan to make a national park out of its last wild river.)

Given the unprecedented size of the flood and its downstream impacts, Wu maintains that the flood he has identified is the deluge of legend.

“The Xia, the great flood, and the control of it have been taken as truth for more than 2,500 years in China,” Wu says. “Now, we have provided the scientific evidence for the flood—[which] means that other texts about the Xia dynasty should be reliable.”

Writing (Not) on the Wall

But as influential as the flood assuredly was to ancient China, the new evidence will not—and, to some scholars, cannot—end the debate over whether the Xia existed.

View Images More skeletons at the Lajia site. The earthquake that killed the residents of Lajia may have caused the landslide that created the dam. Photograph by Cai Linhai

“It’s very important to recognize that this evidence is useful in understanding the course of the Chinese nation,” says Sarah Allan of Dartmouth College, an expert on ancient China who wasn’t involved with the study. However, “their assumption is that even if it’s been revised and changed, [the great flood story] has a historical kernel, and I don’t think that’s necessarily true,” she says.

Allan and other scholars contend that the great flood is best understood as a creation myth that later dynasties used to legitimize their rules. In an influential book explaining the theory, Allan writes that the Shang dynasty mythologized themselves as sun-kings fighting against the Xia, while the Zhou dynasty used the stories as historical precedent to justify conquering the Shang.

While she applauds the researchers for identifying the flood, Allan says that the Xia’s historicity won’t be resolved until examples of Xia-era Chinese writing emerge—a problem that the new flood evidence can’t fix.

“They’re arguing that it was history turned into myth,” she says. “I’m arguing that it’s myth turned into history.”