The Mekong river section in Sang Khom district in Nong Khai province, where it has also recently turned to crystal clear with algae booming, disrupting local fishing there. Credit: Ormbun Thipsuna/ Rak Mekong Group

Mekong’s fine sediments are no longer present: MRC

The Mekong River governing body, Mekong River Commission, has confirmed that the current blue-green hue of the Mekong’s lower section is repeating the phenomenon first appeared in late 2019, with factors contributing to it, “just like in 2019”, and the worrying trend is it is likely to spread to other stretches of the Mekong, where low flows are experienced

The organisation’s environment management and technical support team has analysed the situation and learned that the phenomenon evidently occurring in the Northeast of Thailand is repeating the one that occurred in late 2019, with low flows, a slow drop in river sediments, and algae on the river bottom being present.

Its preliminary analysis of the phenomenon shows that “just like in 2019”, several factors have played a role, the team has pointed, upon the release of its analysis today.

The analysis has indicated that the low flows now being experienced have changed the water colour.

It has further noted that the fine sediments normally found in the fast flowing and deeper water that give the river its brownish appearance are no longer present, creating clearer water conditions.

When sunlight hits the river, the clearer water absorbs what are known as “long-wavelength colours” at the red end of the light spectrum, which gives the river its blue-green hue, the team explained.

Such clearer water then allows microscopic plants or algae to grow on the sand and bedrock river bottom turning the margins of the river green. Algae is normally flushed away by the river current, but due to the low water levels, it has accumulated in certain sections of the river, the team has pointed. 

“Just like the situation in 2019, today’s blue-green water phenomenon is likely to spread to other stretches of the Mekong, where low flows are experienced,” said the MRC Secretariat’s Chief Environment Management Officer, Dr. So Nam. 

Dr. Nam has further projected potential impacts of such the higher water clarity based on the analysis, saying changes in the productivity of the river with less food available for aquatic insects, invertebrates, and small fishes are likely. This will, in turn, affect the productivity of aquatic biodiversity, reducing fish catches and threatening the livelihoods of local communities, he said.

Dr. Nam said the Mekong’s blue-green appearance may persist until flows increase with the onset of the next flood season, which usually begins in late May.

Normal conditions may be restored if large volumes of water are released from storage reservoirs in the Upper Mekong (Lancang) dams and tributary dams, which would mobilise sediments and return the Mekong to its typically brown appearance, he remarked.

River bottoms and sand bars emerge out of the dropping levels of the clear Mekong River in Sang Khom district, Nong Khai province.
Credit: Ormbun Thipsuna/ Rak Mekong Group

Worrying levels

Following the phenomenon, the team has looked at the causes of the clear water and low flows in particular.

It has found that water levels in the river between Jinghong hydropower station in China’s Yunnan province and the Mekong Delta in Viet Nam have fallen to “worrying levels”.

The team has noted that water levels have dropped considerably since the beginning of the year due to lower rainfall, flow changes upstream, hydropower operations in the Mekong tributaries, and outflow restrictions from the Jinghong dam. 

Dr. Winai Wangpimool, Director of the MRC Secretariat’s Technical Support Division said that there have been sudden rises and falls in water levels immediately downstream of Jinghong and further down to Vientiane, which has been challenging for authorities and communities to prepare for and respond to possible impacts.

The MRC’s monthly rainfall observations show that since November last year rainfall has been consistently lower than average, falling by 25%. The organisation’s observed water level data also indicates that outflow from Jinghong dam in China is still relatively low, compared with the promise to return to normality since its grid maintenance in January.

In early January, China’s Ministry of Water Resources notified the four Lower Mekong countries that the outflow from Jinghong would be restricted to 1,000 cubic meter per second from Jan 5 to 24 due to its grid maintenance. The Ministry, however, did not specify the river water level before the outflow restriction nor the volume to be restored on Jan 25. 

Since the initial fall on Jan 1, outflow levels at Jinghong during the first week were stable at 785 cu m/s. It then gradually rose to 1,400 cu m/s on Jan 15, representing a 1.07 metre rise in the water level. But the outflow then dropped to 740 cu m/s during Jan 15-23. It then rose again on Jan 29 to 990 cu m/s, before falling back gradually, reaching 800 cu m/s on Feb 11. 

As of yesterday, the organisation’s water level measuring stations upstream reported the dam’s outflow stood at 775 cu m/s, a plunge of almost half of its normal level of approximately 1,400 cu m/s, which was last recorded in December, the organisation noted.

Last year, China agreed to share year-round water level and rainfall data with the MRC. By the agreement, China pledged to notify the organisation and its member countries of any abnormal rise or fall in water levels or discharge, and provide relevant information on factors that might lead to sudden flooding. 

“Continuing this flow pattern could have an impact on river transport, fish migration, agriculture and river weed collection,” Dr. Winai said. “To help the Lower Mekong countries manage risks more effectively, we call on China and the Lower Mekong countries themselves to share their water release plans with us.” 

At Chiang Saen, the bordering town in Chiang Rai province, and the first monitoring station on the Mekong River in Thailand 300 km away from Jinghong dam, the MRC’s water monitoring station saw water levels drop drastically by about 1 metre during Jan 2-4. Since then, the water level there has been fluctuating between -0.24 m and 0.29 m. 

From Chiang Khan in Thailand to Vientiane in Lao PDR, river levels have been fluctuating between -0.32 m and 0.50 m.  

In the Mekong mainstream, from Nakhon Phanom, Mukdahan and Khong Chiam in Thailand, to Thakhek, Savannakhet and Pakse in Lao PDR, water levels have decreased from 0.04 metres to 0.08 metres, the organization reported.

River levels from Kompong Cham, Neak Luong, Bassac at Phnom Penh, and Prek Kdam have been declining steadily and have remained lower than their long-term averages since early November. Since January, they have seen average daily falls of 0.20 m. 

In the Viet Nam Delta area, from Tan Chau on the Mekong River and Chau Doc on the Bassac River, although frequently influenced by the daily tidal effects of the sea, water levels at the two stations have been fluctuating below and above their long-term averages since November last year, the MRC has noted.

The color turning phenomenon of the Mekong River and its abnormality this year has been locally observing and reporting by some locals living along the river since mid-January. This was then confirmed this week by Thailand’s space development agency, GISTDA, which has released the satellite photographs showing the water color of the Mekong River that has turned to blue-green. (Read: Mekong turns blue, again)

Since, there were no clear explanations from related organisations or agencies until the MRC has released its preliminary analysis today.

However, the organization has not addressed any contribution from another dam in operation on the Lower Mekong, the Xayaburi in Lao PDR’s Xayaburi province, which is being speculated as being part of the causes of these unusual phenomena, nor cumulative transboundary causes and effects, which are critical to predictions and projections of any changes or phenomena in the river.

No explanations have been given from the organisation so far.

Credit: MRC