Why China’s Yellow River is so yellow – and why it’s prone to flooding

The mighty Yellow River has earned the name “China’s sorrow” for its tendency to flood, with devastating consequences, over the centuries.

Now an international group of scientists say they have found the reason why so much sediment builds up in the river over such a long distance – giving it its characteristic yellow tinge and causing frequent overflows – and they are offering their findings as a way to improve the planning, construction and management of river engineering projects both in China and overseas.

The river – the world’s sixth-longest and China’s second-longest and whose basin was the birthplace of Chinese civilisation – collects most of the sediment when it passes through the Loess Plateau in central China.

The new study, led by Hongbo Ma of the department of earth science at Rice University in Houston, Texas, and published on Friday in the journal Science Advances, found that the river actually transports 10 to 20 times more sediment than limits described by existing physical models.

This is because the sediment itself is in small, fine grains, meaning it can travel a long distance because its interaction with the water, banks and riverbed produces minimum friction. The large amount of sediment is what gives the river its yellow colour.

“In typical lowland sand-bed rivers – like the Amazon, the Mississippi, you name it – only about 40 to 60 per cent of the energy is used to transport sediment downstream,” said Jeffrey Nittrouer, a Rice University sedimentologist and a co-author of the paper.

“In the Yellow River, well over 95 per cent of the energy is available to move sediment,” he said.

The physical properties of fine sediment were poorly understood in river science, in part because few other waterways in the world had as much as the Yellow River. The researchers created a new model for sediment movement that they say will be a big help in flood management projects.

The Chinese government has built numerous dams on the river over the years to try and mitigate the flooding, and since 2002 these dams have been releasing huge amounts of clear water once a year to wash the sediment away.

The method was effective in the early years, in some locations lowering the river bed nearly two metres from historical elevations. But the new study found that dams may no longer be a suitable way to manage the river because they reduce its ability to transport sediment by trapping most of it in their reservoirs.

“On the top of the riverbed was mostly fine sediment, perfect for transportation,” said Wu Baosheng, another author of the paper, from the department of hydraulic engineering at Tsinghua University.

But in recent years the riverbed had become coarse with larger, heavier particles that formed a protective layer, Wu said.The finding has added to the debate over dams sparked by the problems at the Sanmenxia Dam in Henan province, the first dam built on the Yellow River in 1960s, which had to be redesigned because of sediment accumulation in its reservoir. The problem continued even after the redesign and several leading engineers have called for the dam to be abandoned.

The researchers said the new model would shed new light on the debate.

“The impact of constructing or removing a dam in terms of sediment transport and subsequent stability of the channel may be evaluated with our new sediment transport model and previous knowledge of the grain size distribution of bed sediment,” the authors said in the paper.