Tonle Sap Lake, which is experiencing “exceptionally low” flows of the Mekong. credit: MRC

“Exceptionally low” flows in the Mekong River pose serious threat to SE Asia’s largest lake of Tonle Sap

Mekong countries are being urged to share data in regard to their dam operations as they are being suspected to contribute to these exceptionally low flows of the river, besides weather anomaly

The Mekong River has experienced the situation called by the regional water regulating body of Mekong River Commission as the ”exceptionally low” flows for the second consecutive year as the body’s latest hydrological situation report has concluded.

”Dry season flows in the Mekong mainstream over the last two years can therefore be characterised as ‘exceptionally low’, ” the report notes.

The report has explored hydrological conditions in the LMB during January-July 2020, following the unusual drops of water levels since mid last year, one of which hit the lowest on record since 1992, which was the year with the lowest flow on record of the Mekong’s late dry season and early flood season.

The low flows have caused Tonle Sap Lake to experience “extremely dry conditions”, with reverse flows at their lowest on record since 1997, the report notes.

Located in Cambodia, the lake is the largest and most productive inland lake in Southeast Asia and considered one of the world’s biodiversity hotspots, and the Mekong’s main fish factory.

The reverse flows into the lake usually occur from mid-May to mid-October, lasting for 120 days on average.

However, this year, reverse flows at normal volumes have not yet occurred, although two extremely small instances were experienced in July, the report pointed.

The report further addresses that the current low flows could have severe impacts on Cambodia due to a loss of fisheries and irrigation potential.

There could be an ecological imbalance due to significant changes of timing, duration and extent of inundations in the Tonle Sap Lake and its surrounding floodplains. This could in turn lead to reduced reproduction through a loss of nursery areas, affecting the growth of fish and other aquatic animals and plants.

A reduction in fish catch is predicted, threatening food security for the most vulnerable communities in the region, according to the report.

In addition, agricultural productivity in Lao PDR and Thailand may also be affected, while further downstream, Viet Nam may experience reduced productivity in the Delta rice bowl.

The likely causes

The MRC’s report preliminarily identifies multiple possible causes of the low flows and drought this year.

Abnormal low rainfall affected by the El Nino weather phenomenon are cited, along with lower water flow contributions from the Mekong tributaries.

More critically, the report also cites ”prolonged low flow conditions” carried over from 2019.

And it also adds that operations of the mainstream dams in the Upper Mekong Basin and the tributary dams in the LMB may have affected the present low flow conditions, using available monitoring, rainfall and weather data from the MRC and other global sources.

However, the MRC said its Secretariat has been unable to obtain official data and information to verify the potential impacts of storage operations in the upper basin.

So, it has asked all Mekong Countries including China to share data and information in regard to their dam operations on the river for more thorough assessment of the situation.

“We call on the six Mekong countries to increase data and information sharing on their dam and water infrastructure operations in a transparent and speedy manner with the MRC. It is time to walk the talk and to act in the common interest of the entire Mekong River Basin and the affected communities,” said Dr. An Pich Hatda, the MRC Secretariat’s Chief Executive Officer.

Last year’s low flows

In mid last year, the Mekong water levels during the early flood season, from June to July, experienced an unusual drop below the historical long-term minimum levels.

The MRC noted that from the upper reaches of the lower Mekong basin in Thailand’s Chiang Saen to Lao PDR’s Luang Prabang and Vientiane and further down to Thailand’s Nong Khai and Cambodia’s Neak Luong, “the water levels are all below those that occurred in 1992, which was by far the year with the lowest flow on record.”

For example, the water level in Chiang Saenat that time was measured at 2.10 meters, which was 3.02 m lower than its long-term average_the average water level measured over 57 years (1961-2018) over the same period. That was about 0.75 m lower than the minimum level ever recorded.

The MRC analysed the situation and learned that some key factors had contributed to the state of what was described as the “regional low flow” of the Mekong river basin, including the very deficient rainfall over the Mekong basin since the beginning of this year. It also cited the deficient groundwater in the region.

Last but not least was the amount of water flowing from the upper part of the basin, where the Mekong is known as Lancang, which was cited as possibly being a potential contribution of the low flow.

Reports of dam maintenance of Chinese dams in the upper part of the river as well as dam operations of the Xayaburi dam in the lower section were communicated throughout the rest of the year, prompting the organisation to pay particular attention to their effects on the river, as the river eventually turned blue due to an absence of sediments trapped by those dams.

As notified by China, for instance, from 5 to 19 July (last year) the amount of water flowing out from the Jinghong dam in Yunnan province would be fluctuating from 1,050-1,250 cubic meters per second (m3/s) to 504-600 m3/s due to “grid maintenance”.