VietNamNet Bridge - The rapid decrease of the sediment load in the Mekong River has been largely caused by hydropower dams. The World Wildlife Fund (WWF) in Vietnam last week said suspended sediment load decreased from 160 million tons to 75 million tons between 1992 and 2014.

 

Dr. Dao Trong Tu, advisor to the Vietnam River Network, while expressing his deep concern over the sharp decrease of 50 percent in sediment on Mekong, pointed out that this was the anticipated consequence of too many hydropower dams on the upper course of the Mekong River.

China has built five large dams on the Mekong, while the number of dams is expected to increase to 12 by 2020. Laos is building Xayaburi and planning to build nine others in the future, while Cambodia is building two dams and hundreds of hydropower and irrigation reservoirs on Mekong’s branches.

The sediment load is 150-170 million tons a year on the Mekong. In the first 10 years of Manwan dam’s operation, the main stream of the Mekong lost 20 million cubic meters of sediment.

“The 50 percent fall after 20 years is too serious,” Tu said, adding that it is the large dams which retain sediment in the upper course, thus leading to a sharp fall in sediment in the lower area.

Scientists warmed about this many years ago, before the dams were built. 

“The danger from the sediment decrease is obvious. All of us will suffer when the climate changes,” Tu said.

Erosion

According to Tu, Vietnam’s Mekong River Delta receives 79 million tons of sediment a year. Of this, 9-13 million tons of deposit are on the floodplain, while the remaining expand the land and make coastal aquaculture areas more fertile.

The deposit of sediment in shallow coastal waters helps protect the coast from erosion by waves. Agriculture and fisheries of Vietnam aquaculture depend on the transport of sediment’s nutrients.

Tu fears that the decrease in the sediment load will force Vietnamese to pay more for agriculture and aquaculture. 

Meanwhile, the sediment decrease would lead to land erosion in the coastal areas.

“The process would be even more serious in the context of the seawater level rise caused by climate change,” Tu said.

Tu cited official reports as saying that about 1 million people will bear direct consequences from land erosion and land loss in Mekong River Delta. 

Dr. Le Phat Quoi from the HCM City National University has also expressed worries about the construction of dams on the Mekong.

“The sediment decrease will affect land improvement, cause land erosion and bring danger to the fisheries,” he said.

Dat Viet