River geomorphology: Divided by five | The Economist

BY BUILDING dams in the Himalayas, Chinese engineers are tinkering with one of the world’s great sets of watersheds. Five great streams—the Red River, the Yangzi, the Irrawaddy, the Salween and the Mekong—flow within about 180 kilometres (110 miles) of one another from the south-eastern edge of the Tibetan plateau, separated by parallel mountain ranges, before charging off to empty eventually in different seas. Yet research suggests that before this plateau began to rise, 50m years ago, at least two of these streams, and possibly more, were one.

The evidence, gathered by Peter Clift of Louisiana State University and Zheng Hongbo of Nanjing Normal University, comes from examination of the Red River delta in the Gulf of Tonkin in Vietnam and the middle reaches of the Yangzi. The notion that the Red River delta was the mother estuary of East Asia was conceived a decade ago by Marin Clark, then at the California Institute of Technology, and Mr Clift believes that he has confirmed it. By analysing data collected by the oil industry, he has found that the volume of sediment in the delta is far greater than it should be, given the Red River’s relatively modest size today.

He has also shown, by studying isotopes of a metal called neodymium, that much of this sediment, from the Himalayas, once came from a block of crust known as the Yangzi craton, which underlies Yunnan and Sichuan. The ratio of two isotopes of neodymium differs between rocks from these two places. Between 45m and 23m years ago it shifted from one rich in Yangzi-craton rocks to one dominated by Himalayan rocks as now.

That shift, Mr Clift thinks, marks the gradual separation of the Red and Yangzi rivers, and in particular the formation of a feature, famous in China, called the First Bend of the Yangzi. This bend, in Yunnan, is a place where the river makes a U-turn from flowing south to flowing north-east. The dating of 23m years is also confirmed by the fingerprint of a second metal, zirconium. This, too, varies from one river basin to another. Mr Zheng has found that deep alluvial beds in the Yangzi’s middle course carried the zirconium signature of modern Yangzi sediment even 23m years ago.

via River geomorphology: Divided by five | The Economist.


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