Mekong River drops to “worrying” levels, some sections turning blue-green

12 Feb 21 Mekong River Commission Source  Vientiane, Lao PDR, 12 February 2021 — Mekong River water levels between Jinghong hydropower station in China's Yunnan province and the Mekong Delta in Viet Nam have fallen to worrying levels, the Mekong River Commission (MRC) Secretariat said today.

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.

"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," said Dr Winai Wangpimool, Director of the MRC Secretariat's Technical Support Division.

The MRC's monthly rainfall observations show that since November last year rainfall has been consistently lower than average, falling by 25%. The Commission's observed water level data also indicate that outflow at the Jinghong station yesterday was 775 cubic meters per second (m³/s), a plunge of almost half of its normal level of approximately 1,400 m³/s, which was last recorded in December.

Since the initial fall on 1 January, outflow levels at Jinghong from 1 to 7 January were stable at 785 m³/s but gradually rose to 1,400 m³/s on 15 January, representing a 1.07-metre rise in the water level. The outflow then dropped to 740 m³/s during 15–23 January, before rising to 990 m³/s on 29 January. It then fell gradually, reaching 800 m³/s on 11 February.

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 m³/s from 5 to 24 January due to the maintenance of power grid transmission lines. The Ministry, however, did not specify the river water level before the outflow restriction nor the volume to be restored on 25 January.

Last year, China agreed to share year-round water level and rainfall data with the MRC. By agreement with the MRC, China pledged to notify the MRC 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."

Chiang Saen – the first monitoring station on the Mekong River in Thailand located approximately 300 km from Jinghong – saw water levels drop drastically by about 1 metre during 2–4 January. Since then, the water level at Chiang Saen 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 decreased from 0.04 metres to 0.08 metres.

Further downstream in Cambodia, water levels along the Mekong in Stung Treng and Kratie fluctuated between -0.02 m and 0.05 m. However, the levels remain higher than the long-term average.

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.

The Viet Nam Delta area – from Tan Chau on the Mekong River and Chau Doc on the Bassac River – is 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 Mekong River takes on an aquamarine hue

Due to the low flows, a slow drop in river sediments, and the presence of algae on the river bottom, the Mekong in Thailand's north-eastern province Nakhon Phanom has recently begun to acquire an aquamarine colour – a phenomenon that occurred once in late 2019.

An MRC preliminary analysis of the phenomenon that looked at the causes and potential impacts showed that, just like in 2019, several factors played a role.

The analysis indicated that the low flows now being experienced have changed the water colour. 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 clearer water allows microscopic plants or algae to grow on the sand and bedrock river bottom turning the margins of the river green. Algae are normally flushed away by the river current, but due to the low water levels it has accumulated in certain sections of the river.

"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.

Citing the analysis, Dr Nam added that some of the potential impacts of higher water clarity include changes in the productivity of the river with less food available for aquatic insects, invertebrates, and small fishes. This will in turn affect the productivity of aquatic biodiversity, reducing fish catches and threatening the livelihoods of local communities.

Dr Nam noted that 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.

The Mekong River and its tributaries support nearly 70 million people in the Lower Basin, providing livelihoods, food security and rich ecosystem services. As economic growth continues to make increasing demands on the river, the threat to riverine communities is being compounded by a growing population, hydropower and infrastructure development, climate change, flooding and drought.

Note to editors:

The MRC is an intergovernmental organisation for regional dialogue and cooperation in the lower Mekong river basin, established in 1995 based on the Mekong Agreement between Cambodia, Lao PDR, Thailand, and Viet Nam. The organisation serves as a regional platform for water diplomacy as well as a knowledge hub of water resources management for the sustainable development of the region.


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