Trinh Xuan Hoang, head of the Institute of Water Resources Planning 's Environmental Planning Unit said the alluvial deposits in the river began to decrease after the Hoa Binh and Tuyen Quang reservoirs were built.

From 1988 to 2008, the amount of mud and sand in Son Tay town, 42km away from central Hanoi , was estimated to reach 45 millions tonnes annually. In central Hanoi , the volume reached o­nly 38 million tonnes annually.

Dr Dao Trong Tu, former Deputy General Secretary of the Vietnam Rivers Network, said although alluvium didn't play a vital role in agricultural production in this region as it did in the Cuu Long ( Mekong ) Delta, farmers could still suffer from losses caused by sudden changes in river water quality.

For example, shrimp or fish that had become accustomed to living in muddy water might have problems with clear water, he said. Moreover, a shortage of alluvium could lead to massive changes in the environment and break the ecological balance.

Experts said that all of the country's hydroelectric power plants were equipped with flood gates which trapped large amounts of stagnant alluvium. There have been no effective measures taken to return the alluvium to the river.

A lack of specific figures o­n alluvium in Hong River , as well as other rivers in the country, was also contributing to the difficulties, they said. There are currently o­nly six stations that measure alluvium o­nce per month.

A redesign of flood gates at hydro-electric power plants along the river would temporarily improve the situation, Hoang said.

Re-examination of the effects of hydro-electric power plants o­n the rives and establishment of more measuring stations could help automatically update data o­n alluvium and other elements in the water, he said.

Based o­n the results, more drastic measures could be developed to improve the situation.

Hong River is o­ne of the largest rivers in the north of Vietnam and carries a large amount of alluvium for the region's agricultural production. The region is widely considered as the country's second granary.

However, alarms have been raised over pollution in the river since 2009. Since then, more than 104 species of fish have been impacted by poor water quality, notably moi co (clupanodon thrissa) and chay (tenualosa) fish which are listed as rare species in Vietnam 's Red Book.