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 on 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 resulting from increasing sand filling - a chronic problem plaguing most reservoirs on high sediment-laden rivers, according to the report.

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 its flood discharge capacity.

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, 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 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 global concern concerning land erosion, desertification, sediment yield, transport and deposition in reservoirs and lakes, river flows, and estuary and coastal interactions between sediment and hydropower projects, experts warned Tuesday.

(China Daily October 20, 2004)