The build-up of silt, a key problem threatening the huge Three Gorges Reservoir, the largest one of its kind on the Yangtze River, is under control.

About 40 percent of sediment flowing into the reservoir at its dam site has been washed away, with the amount of remaining suspended silt getting smaller.

Annual sediment passing through the dam site totals 530 million tons, and has been reduced to 200 million tons, China Three Gorges Project Corporation announced Tuesday.

"Most sediment can be discharged from the reservoir to ensure its long-term ambition of controlling floods, improving navigation and generating hydropower," the corporation said at the release of its latest monitoring report Tuesday during the ongoing ninth International Symposium on River Sedimentation (ISRS).

One effective way is to lower water levels and flush away the higher sediment brought into the reservoir during flood periods by releasing floodwater through the huge sluice gates at the bottom of the dam between June and September -- the peak period of summer floods.

By the end of the flood season in October, the huge reservoir then stops releasing floodwater and starts to store water with a low sediment content to generate hydropower and improve navigation up and down the dam site.

In the past, the sediment discharge operation has enabled effective operation of the Three Gorges reservoir and prevented drastic shrinkage of its water storage capacity resulted from increasing sand filling -- a chronic problem plaguing most reservoirs on high sediment-laden rivers, according to the report.

Meanwhile, to reduce sediment upstream, a massive water and soil conservation program has been launched to rehabilitate the ecosystem of the Yangtze River.

Over the past two to three decades, a score of major reservoirs were built along major tributaries upstream of the Yangtze to prevent sediment from entering the Three Gorges Reservoir.

In the next 10 years, a group of large reservoirs and key hydropower stations are scheduled to be completed upstream of the reservoir to further reduce sediment from flowing downstream, experts said.

Since 2002 three projects have been launched in the north to prevent the further rising of the Yellow River's bed which has risen 10 meters above its levees for hundreds of kilometers in the Henan and Shandong sections of its lower reaches.

During these operations, high-sediment laden waters were released from key reservoirs upstream in man-made waves to flush away millions of tons of sand downstream to deepen the river bed and enlarge the flood discharge capacity.

Such experiments, the only ones of their kind ever done, have been proven effective for the Yellow River -- the world's muddiest river -- to keep its balance between water and sediment and sustainability of its ecosystem.

These major achievements in sediment control and reduction have been submitted to the ongoing ninth ISRS for information exchange.

Sponsored by the Ministry of Water Resources and supported by the United Nations Educational, Scientific and Cultural Organization (UNESCO), some 500 Chinese and foreign experts are attending the symposium to share their latest research and achievements made all over the world in erosion control and river-related sedimentation reduction.

To further promote international cooperation in the field, all delegates applauded the symposium's decision for the official inauguration of the World Association for Sediment and Erosion Research, which was also announced Tuesday.

Experts and officials from UNESCO said they were confident that association and cooperation with it can further improve global conservation, utilization of soil and water resources and the betterment of ecosystems along the world's river basins.

Sedimentation problems are a matter of global concern as they include issues arising from land erosion, desertification, sediment yield, transport and deposition in reservoirs and lakes, process of river course, estuary and coast and interactions between sediment and hydropower projects, experts warned Tuesday.

According to preliminary statistics, the annual erosion of surface soil from global river basins amounts to 60 billion tons with 5 to 7 million hectares of farmland lost each year.

About 1 percent of the precious storage capacity of the world's reservoirs is annually lost due to river-related sedimentation with more floods and droughts induced and ecosystems deteriorated as a result.

Experts attending the seminar made it clear that the problem has become a global challenge with adverse impacts on the worst problems facing humankind this century -- rapid population increase, a worsening shortage of resources and the rehabilitation of ecosystems.

(China Daily October 20, 2004)