The Mekong River at Sangthong district  1

Mekong River’s aquamarine hue likely to occur elsewhere due to low flows, bringing possible risks

Vientiane, Lao PDR, 09 Dec 2019

Vientiane, Lao PDR, 9 December 2019The extremely low flow, slow drop in the river sediments, and presence of algae on the sand and bedrock river bottom are some of the possible causes why the Mekong River has recently acquired an aquamarine color, says the Mekong River Commission. This phenomenon may spread to other parts of the River where low flows occur. 

The Mekong River at Thailand’s Nakhon Phanom and Lao PDR’s Thakhek has lately taken on a blue-green hue. According to the MRC’s preliminary analysis that looked at the causes and potential impacts of the phenomenon, many factors have contributed to the occurrence whose consequences could be problematic.   

The analysis has indicated that the extremely low flows now being experienced in the Mekong, during one of the worst droughts ever experienced in the region, have changed the water color. The fine sediments normally found in the fast flowing water and deeper water levels that give the water the brownish color have dropped out, creating clearer water conditions. 

This means that, added the analysis, when the sunlight hits the river, the water strongly absorbs what is known as the “long-wavelength colors” at the red end of the light spectrum and makes the river look blue. This occurs in just a few meters of water.  

The Mekong River at Sangthong district  2 v2

The Mekong River at Sangthong district of Lao capital city Vientiane is seen from the sky, indicating the area’s low water level and the algae that are building up at the very shallow part of the river.


The clearer water allows microscopic plants or algae to grow on the sand and bedrock river bottom causing the margins of the river to turn green, noted the analysis. These algae are normally flushed away by the river current. But due to the exceptionally low river level, this is not happening at Nakhon Phanom and perhaps elsewhere. Instead, the algae are building up. 

The analysis warned that the conditions could probably be made worse if fertilizers used in agriculture enters the river feeding the growth of algae. 

“The blue-green water phenomenon is likely to spread to other stretches of the Mekong River where low flows are encountered,” said the MRC Secretariat’s Chief Environment Management Officer Dr. So Nam, who led the analysis. “The issues of low flows and sedimentation could possibly lead to adverse impacts that have been well-quantified in the MRC Council Study.” 

According to the analysis, some of the potential impacts include change in productivity of the river with less foods available for insects and small fishes, and reduced productivity of aquatic biodiversity, including fish due to high water clarity. This will in turn affect the fish catches and the livelihoods of local communities. 

High water clarity also leads to significant algal growth or algal bloom. During the day, this leads to high dissolved oxygen levels, but to very low dissolved oxygen at night. This can affect lives of different fish, according to the analysis. 

The analysis also added that if water clarity remains high, the algal bloom can cause thick green mats that will rot and smell badly. It warned that if the conditions get worse, the algae can change from green species (Chlorophyceae) to blue green species (Cyanophyceae), producing toxic substances that can harm animals.  

But Mr. Nam said that “it is unlikely such conditions will occur in the main river, and be restricted to backwaters.” However, he warned that “people should be careful watering their animals if the water is very green”. 

Based on the analysis, the present conditions of blue-green appearance in the Mekong may persist until flows in the river increase at the onset of the next flood season, which usually begins in late May. The conditions may also recuperate, added the analysis, if large volumes of water are released from the storage reservoirs in the Upper Mekong (Lancang) dams and tributary dams to mobilize sediments and give the Mekong its typically brown color. 

The MRC’s flow monitoring shows that dry season flows have increased during the past few years due to water release from reservoirs in order to produce electricity. 

Read the news in Khmer or Vietnamese.


Note to editors:  

The MRC is an intergovernmental organization 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 organization 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.


- END - 

For more information, please contact:
Mr. Sopheak Meas
Communication Officer for Press
Mekong River Commission Secretariat
Telephone: +856-21-263263, ext. 4012
Mobile: +856-20-77779168



Latest News

MRC's 25th Anniversary: A time to look back and see what lies next

It has been 25 years since Cambodia, Laos, Thailand and Vietnam signed on 5 April 1995 the Mekong Agreement to develop and protect the Southeast Asia’s mighty Mekong River.

Campaign launched to celebrate 25th Anniversary of MRC, inspire river protection

The MRC has launched a social media campaign in which all nationalities can enter and stand a chance to win different prizes. The campaign, entitled Mekong 2030, aims to

Japan provides $3.9 million to tackle Mekong flood and drought issues

The government of Japan today granted JPY 412 million or approximately US$3.9 million to the Mekong River Commission in advancing its flood and drought monitoring and forecasting functions.

Safeguarding livelihoods for fishing communities in Mekong Countries

The community at Dun Ei, a fishing village 180 kilometers northwest of Phnom Penh, has been making a living through fishing along the Pursat River for generations...

Pilot program to monitor impacts from Xayaburi and Don Sahong takes off

Officials and experts from the lower Mekong countries kicked off the Mekong River Commission’s pilot program to monitor transboundary environmental impacts from the Xayaburi and Don Sahong dams to advise on measures to address any negative effects from th

Media Contact

For media inquires, please contact:
Sopheak Meas:
Le Thi Huong Lien: or or view the Contact page here