China's largest freshwater lake dries up

Harold Thibault in the Guardian (UK): For visitors expecting to see China's largest freshwater lake, Poyang is a desolate spectacle. Under normal circumstances it covers 3,500 sq km, but last month only 200 sq km were underwater. A dried-out plain stretches as far as the eye can see, leaving a pagoda perched on top of a hillock that is usually a little island. Wrapped in the mist characteristic of the lower reaches of the Yangtze river, the barges are moored close to the quayside beside a pitiful trickle of water. There is no work for the fisheries.

According to the state news agency Xinhua, the drought – the worst for 60 years – is due to the lack of rainfall in the area round Poyang and its tributaries. Poor weather conditions this year are partly responsible. But putting the blame on them overlooks the role played by the colossal Three Gorges reservoir, 500km upstream. The cause and effect is still not officially recognised, even if the government did admit last May that the planet's biggest dam had given rise to "problems that need to be solved very urgently".

"Every year, when the Three Gorges reservoir stores water – to power the dam's turbines during the winter – the flow rate in the Yangtze drops. This in turn increases the rate at which the level of Poyang lake falls, and the period of low water comes sooner," said Ye Xuchun, a researcher at China's Southwest University. In partnership with scientists at the Lake Science and Environment laboratory at Nanking University, he has published a comparative analysis of water levels in the Three Gorges basin and at the lake's northern extremity, near the city of Hukou, where the outflow from Poyang joins the Yangtze.

The authors conclude that the artificial regulation of the reservoir, which must be kept full to optimise electricity output, reduces the water level in the lower reaches of the Yangtze. This means that the big river no longer "plugs" the lake's northern outlet, so the other rivers feeding into Poyang simply pass through the dwindling lake and run on downstream...

Lake Poyang, seen from a NASA satellite