Dwellings on the Tonle Sap lake in Siem Reap, Cambodia. (Photo: Wikipedia)
The annual reversal of the river flow in the Tonle Sap has failed to arrive amid a long-running drought with water levels now “very low,” the Mekong River Commission (MRC) said in its weekly report.
As a major tributary of the Mekong River, the Tonle Sap stretches from Phnom Penh to Siem Reap and forms a giant inland lake during the wet season. At its peak, the river reverses course and empties into the South China Sea through Vietnam.
But a drought, now in its second year, coupled with upstream dams has resulted in water levels hitting record lows across the Lower Mekong Basin, which up to 70 million people rely on for their livelihoods.
The MRC Flood and Drought Management Centre in Phnom Penh said low inflows from the Mekong and low rainfall in the Tonle Sap's upper catchment areas had “resulted in a very critical situation” and delayed the reversal.
“More than half of the annual inflow to the lake originates from the Mekong mainstream. Thus, flow alterations in the mainstream would have direct impacts on the Tonle Sap water levels and hydrology,” it said.
Mekong water levels in Stung Treng, Kratie, Kampong Cham and Neak Luong had risen from the previous week but three months into the current wet season were "still below their minimum levels” recorded between 1960 and 2019.
It said water levels on the Bassac River at Chaktomuk and Koh Khel as well as the Tonle Sap River at Phnom Penh Port and Prekdam had also risen but were also below minimum levels not seen over the last six decades.
Rainfall in May, June and July was about 70 percent below last year when the drought was already underway with water levels about two-thirds lower than normal.
The MRC again warned that low waters in the Tonle Sap could affect fish spawning in the surrounding floodplain and lead to water shortages for agricultural production.
Fishermen have complained their daily catches has been reduced to a paltry two or three kilograms and have blamed large-scale dam construction in Laos and China for their plight.
At least 220 dams have been built or are planned for the Mekong and its tributaries, although some observers like the Stimson Centre in the United States say that number is more than 400.
A report by research company Eyes on Earth Inc. in April found that China has been restricting the Mekong’s natural water flow by hoarding water through a cascade of dams.
“When drought sets in, China effectively controls the flow of the river,” Brian Eyler, regional director of the Stimson Centre think tank, said.
Beijing has dismissed the findings and denies any damage caused by Mekong dams on downstream countries.
Scientists have also tied the drought to the climate phenomenon known as the Indian Ocean Dipole, which like El Nino in the Pacific Ocean is aggravated by global warming, impacting on the Indian monsoon and the annual wet season in mainland Southeast Asia.
Unusually cooler than average sea surface temperatures in the eastern half of the Indian Ocean cause less rainfall in Southeast Asia, while warmer waters in its west cause floods in East Africa.