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21 May 2007 09:52
China’s Three Gorges dam, the world’s largest hydropower project, is retaining huge amounts of sediment and nutrients and causing significant erosion in the downstream reaches of the Yangtze River, researchers have found.
In a paper published in the latest volume of the Geophysical Research Letters, Chinese scientists said the dam had retained 151-million tonnes of sediment each year since 2003.
The researchers from the East China Normal University in Shanghai calculated supplies of water and sediment at places along the river which had previously not been monitored and combined them with the regular gauging stations.
“The Three Gorges dam, which has regulated the waters of the Yangtze River since 2003, retains two-thirds of the upstream sediment each year,” they wrote.
“In response to this retention, significant erosion occurs in the riverbed downstream of the dam ... Sediment flux to the Yangtze River mouth has decreased by 31% per year.
The Yangtze delta is shrinking.
“Continued sediment retention at these rates, combined with more dams planned for the watershed, will severely affect people and the ecosystems on the Yangtze delta,” they added.
The paper gave no details nor estimates of how many people would suffer.
Official Chinese press reports say the build-up of silt in the Three Gorges reservoir is under control.
Huge sluice gates at the bottom of the 185m-high dam wall are opened between June and September to lower water levels and flush away sediment collected in the reservoir during floods.
In the past, large damming projects elsewhere have resulted in silt and nutrients being trapped behind the dams, damaging fish stocks and the fertility of downstream farmland.
Many environmentalists say creation of the Three Gorges dam will have unforeseen ecological effects apart from becoming a cesspool of sewage and industrial pollutants.
The reservoir created by the dam has inundated two cities, 11 counties and 116 towns in Hubei province and neighbouring Chongqing municipality.
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