For three decades, China has been building dams on the upper Basin of the Mekong River, worrying countries downstream that China could one day turn off the tap. New data shows that for six months in 2019, while China normal to above average precipitation in most of its part of the Mekong, its dams held back more water than ever — even as downstream countries suffered through an unprecedented wet season drought. These new findings confirm what many had long suspected: China is impounding much more water than it ever has before and is causing erratic and devastating changes in water levels downstream.

New Evidence

How China Turned Off the Tap on the Mekong River

By Brian Eyler Lead Author ·  Regan Kwan Data Visualization ·  Courtney Weatherby Co-author

Editor’s Note, May 8, 2021. This article has been updated to reflect changes in the Eyes on Earth Natural Flow Model made after the discovery of data problems from one of the satellites used to measure surface wetness in the Mekong basin. The correction has produced a much more accurate model of natural river flow and the improved data support this article’s core findings. Read more about how transparency and open data improved our work.

What We Know

  • New data shows that during a severe wet-season drought in the lower Mekong Basin in 2019, China’s dams restricted nearly all upper Mekong wet season flow. Snowmelt and rainfall was normal to high for much of China’s portion of the basin for the entirety of 2019. If China’s dams did not restrict flow, portions of the Mekong along the Thai-Lao border would have experienced significantly higher flows from July 2019 to the end of the year instead of suffering through severe drought conditions.
  • This is part of a long pattern that has driven numerous wet season droughts. The increasing frequency of wet season drought in the Lower Basin tracks closely to the way China releases water during the dry season and restricts water during the wet season.
  • China is impounding much more water than it ever has in the past during wet seasons and releasing more water than ever before during the dry seasons. After the completion of the Nuozhadu dam in 2012, China’s dam operations change significantly with dams collectively impounding more water in the wet season and releasing more water during the dry season.  released.
  • China’s dam management is causing erratic and devastating changes in water levels downstream. Sudden unexpected flood events downstream can now be linked to the completion of the Dachaoshan dam and the Nuozhadu dam in 2002 and 2012-2014. Unexpected dam releases caused rapid rises in river level that have devastated communities downstream, causing millions in damage shocking the river’s ecological processes.
Part of the Mekong Policy Project
Southeast Asia
Graphics Used in this Story

Eyler, Brian and Weatherby, Courtney. “New Evidence: How China Turned off the Tap on the Mekong River”. April 13, 2020. The Stimson Center:


We are grateful to Eyes on Earth for data collaboration. 

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Confirming China’s Dams Drive Mekong Drought

For most of 2019, much of China’s portion of the upper Mekong received higher to average levels of precipitation, yet during the wet season its dams blocked or restricted more water than ever before - even as downstream countries suffered through an unprecedented wet season drought.

In the 1990s when China built the first dam on the upper Mekong, many speculated that China could use its dams to restrict water from the Mekong downstream, effectively turning off the tap for the countries which rely on the Mekong’s provisions for economic stability and security. Today a total of eleven mega-dams dot China’s upper Mekong reaches and collectively store as much water as the Chesapeake Bay. The frequency and severity of downstream drought has increased over the last two decades, and from 2019 to the present, Thailand, Cambodia, and Vietnam have suffered through the worst ongoing drought in history.

The 2020 study from Eyes on Earth uses physical river gauge evidence from the Mekong River Commission and remote sensing processes to definitively confirm long held-concerns that the ongoing drought is related of Chinese water management policy. The Eyes on Earth study shows when China has restricted water from its downstream neighbors, for how long it has restricted this water, and the enormous quantity of water China has restricted over the last three decades.

The report’s most significant finding is that for most of 2019 much of China’s portion of the upper Mekong received high to average levels of precipitation, yet its dams blocked or restricted more water than ever as downstream countries suffered through an unprecedented wet season drought. The restriction came after China’s upstream dams released nearly all of their water between January and June 2019 to produce an unprecedented amount of hydropower for sale to markets in China. The amount of rainfall and snowmelt in China was enough to keep water levels in much of the Lower Mekong above average between June 2019 and March 2020 if China’s dams were not restricting that water.

Now that these findings are available and can be reported in near-real time, it is incumbent upon stakeholders in Lower Mekong countries to pursue changes to the way China’s upstream dams are operated and negotiate for a more equitable distribution of water resources. Working through the Mekong River Commission, a transboundary river basin organization established by the 1995 Mekong Agreement, to achieve these ends is a best path forward.

Click to Zoom
Expected River Level
How much water would be in the Mekong if China's dams didn't alter river flow?
Expected River Level

The water levels naturally ebb and flow as the Mekong transitions from its wet season to the dry season each year.

The water level of the Mekong can be reliably modeled based on upriver rain and snowmelt.

The data, provided by Eyes on Earth, is based on satellite measurements of precipitation in the upper Mekong Basin and snowmelt from the Himalayan mountains. 

This gives researchers a basis to measure the impact of China's upstream dams.

By measuring the difference between expected water level and actual water level, we can see the difference between a natural and dammed Mekong. 

Expected River Level
Actual River Level
Amount above expected
Amount below expected
How much water is actually in the Mekong as it leaves China?
Actual River Level

Actual water levels are measured in Chiang Saen, Thailand, the river gauge closest to the Chinese border. This data is collected by the Mekong River Commission and laid atop the Eyes on Earth model of expected river level at the same point.  

Actual and expected water levels diverge significantly around 2012.

When the lines diverge, it means something, likely dams, is altering the Mekong’s natural flow. If the blue line is above the orange, then less water than expected is flowing through the river. If the orange line is above the blue, then more water than expected is flowing through the river.

The lower Mekong has been much drier than expected for the last three years, with water levels swinging significantly.
Amount below expected

The area chart shows just how far below (or above) the actual water levels are from the expected water levels.

Expected River Level
Actual River Level
Amount above expected
Amount below expected
In 2012, as a major new dam came online, more water has been consistently held back during the wet seasons and released during the dry season, amounts varying year to year.
Actual River Level

The Eyes on Earth data shows that China changed how it managed water levels in 2012, increasing the amount of water it restricted.

This change coincides with completion of the Nuozhadu Dam

The massive new dam first came online in late 2012. Filling a reservoir half the size of the Chesapeake Bay, it is the largest single dam in China’s cascade.

The data shows that more water was consistently held back during the wet seasons.

Expected River Level
Actual River Level
Amount above expected
Amount below expected
In 2019, the lower Mekong experienced a record-setting wet season drought while the upper Mekong basin in China received ample rainfall.

The drought had significant impact: Today some highly populated portions of Vietnam’s Mekong Delta have completely lost access to fresh water. In 2019 and 2020, seventeen provinces along the river declared drought disasters. Food production dropped significantly in Thailand and Vietnam.

Yet during this wet season drought, China's dams held back more water than ever before when it was desperately needed downstream.

Higher to normal levels of rain and snowmelt occurred in much of China’s portion of the upper basin during the traditional monsoon season. 

China’s dams shed nearly all of their storage in the early months of 2019 and when the wet season came, nearly all of that water remained upstream behind China’s dams.

Despite this, China’s Foreign Minister Wang Yi declared that a lack of rain was the main cause of the drought and that China “increased water outflow from the Lancang River to help Mekong countries mitigate the drought.” 

China did increase water flow during the dry season, at unprecedented levels, to generate a massive amount of hydropower. But after June 2019, China’s reservoirs recharged, restricting water from downstream countries while they suffered drought.

Data Sources: Mekong Dam Monitor; Mekong River Commission.

Dams Driving the 2019-2020 Mekong Drought

Despite above normal rainfall in much of the upper Mekong basin, China’s dams held back nearly all of the additional water during a wet season drought that left many without access to fresh water.

During the 2019 monsoon season (June to November) the regional and international media provided wide coverage of record low river levels throughout the lower Mekong countries. In July 2019, Thailand mobilized its military to respond to a drought emergency in its northeast provinces where rocks and islands that are typically covered by the river’s monsoon pulse were exposed at levels never before seen. In Cambodia, fishing communities alongside the Tonle Sap Lake – the world’s largest inland fishery from which Cambodians catch up to 70% of their protein intake – reported fish catches 80-90% lower than usual. Today some highly populated portions of Vietnam’s Mekong Delta have completely lost access to fresh water.

Impact of the 2019 Drought

Drag slider handle to compare.

This comparison shows the extent of the Mekong drought on the Thai/Lao border (top) and the Tonle Sap Lake (bottom) on July 15, 2019 compared to a normal monsoon season on the same date in 2017.

In July 2019, China’s upstream portion of the Mekong was receiving above normal precipitation and snowmelt but that runoff was restricted by China’s 11 mega-dams.

Image Source: Planet Labs Inc.



Image Source: Planet Labs Inc.

“China has overcome its own difficulty and increased water outflow from the Lancang River to help Mekong countries mitigate the drought.”

-Wang Yi, Chinese Foreign Minister to Reuters, Feb 20, 2020

This new data, however, tells a very different story: Normal amounts of rain and snowmelt occurred in much of China’s portion of the upper basin during the traditional monsoon season and nearly all of that water remained upstream behind China’s dams. The chart below shows China’s dams impounding and restricting nearly all of the water that fell in the upper basin while downstream countries suffered and continue to suffer. This level of restriction is unprecedented. China’s rainfall and snowmelt could have done much to alleviate drought and maintain an above- average river level particularly along the Thai/Lao border and during the early months of the wet season drought. Instead the river ran bare. Yet at an early 2020 meeting, China’s Foreign Minister Wang Yi declared that a lack of rain was the main cause of the drought and said China had suffered from it too. Eyes on Earth’s findings prove that statement false. 

Now that the basin is well into the annual dry season, the lack of water in the Mekong system has caused 17 provinces across the Mekong Basin to declare emergency drought conditions. The drought is also causing a major drop in rice production in Thailand and Vietnam, both major suppliers to the regional and global rice market. Much of this is happening as COVID19 locks the region down, delivering even deeper economic wounds to the region.

To date, most of the blame for these drought conditions has been laid upon a prevailing El Nino weather pattern which delivered abnormally lower than usual rainfall to the Mekong countries for most of 2019, the monsoon season included. Indeed the region at large experienced low levels of precipitation and the upstream dam restrictions during the wet-season exacerbated drought conditions along the Mekong mainstream.

China Received Significant Rainfall...
This precipitation map shows that much of the upper Mekong basin received above-average rainfall during the 2019 wet season compared to the previous eighteen years.
Click to zoom.
...while restricting nearly all of it.
Noted below, China's dams held back so much water that they entirely prevented the annual monsoon-driven rise in river level at Chiang Saen, Thailand. This has never happened before since modern records have been kept.
Click to zoom.

Understanding China’s Water Policy Choices

Chinese policymakers consider water a sovereign resource rather than a shared resource, an approach which has significant influence on countries downstream.

Why would China’s dam operators hold back so much water in a time of dire need downstream in Southeast Asia? If China’s choice to restrict unprecedented amounts of water were open knowledge among downstream stakeholders, such a decision would not bode well politically for China. But China treats data about water flow and hydropower operations as a state secret. This lack of transparency allowed China to set a narrative of shared suffering due to the drought and established common cause for China to deepen its economic cooperation with the downstream through its Lancang-Mekong Cooperation Mechanism. 

Dam NameYear of CompletionInstalled Capacity (in megawatts)Water Storage Capacity (in cubic meters)
Miaowei2017 (January)1400660,000,000
Huangdeng2017 (November)19001,613,000,000
Dahuaqiao2018 (February)900293,000,000
Lidi2018 (June)42075,000,000
Wunonglong2018 (December)990284,000,000

The answer to China’s motivations might lie in how China values its water resources. To Beijing, water is considered a sovereign commodity for consumptive use rather than a shared resource to be made available in an equitable manner to downstream stakeholders. Stimson’s researchers have often heard Chinese stakeholders repeat a worrying position: Not one drop of China’s water should be shared without China using it first or without making those downstream pay for it. Notably, China has not signed international treaties for most of its forty transboundary rivers. Additionally, Chinese stakeholders often mention that its portion of the basin contributes an insignificant amount of water to the river system, but during the dry season and during times of drought, China’s upstream contribution to total river flow is more than 40%. Also, China’s upstream impacts are felt more by Thai and Lao communities situated more closely to China.

Ed. Note: Since this feature article was originally released in April 2020, the authors of this article have discovered that the unprecedented wet-season dam restrictions result from an optimization of dry season releases used to generate maximum amounts of electricity for sale to China’s eastern provinces. Electricity market prices are much higher in the dry season than the wet season, so the decision to release water during the dry season is profit-driven. To prepare for the next dry season, China’s dams must restrict flow during the wet season, and 2019 was the first time the entire 11 dam cascade was operated in a coordinated fashion. This came as a result of three new dams coming online during 2018 and the installation of new transmission lines to China’s eastern urban demand markets. Read more.

Additionally, water from the Mekong could be transferred into other basins that lead to China’s east coast, actions that would permanently remove this water from the Mekong, but currently no evidence of this activity exists. Interbasin water transfer projects are difficult-to-impossible to keep secret  due to their size and the number of involved stakeholders. Transfer projects from the Mekong Basin have been proposed, but are currently unlikely due to their technical difficulty and high level of risk. But transfer projects are happening in other major basins in China’s Yunnan province and in other parts of China (e.g. $62bn South-North Water Transfer Project). The idea of pulling water from the Mekong and other transboundary rivers to use domestically likely remains an attractive policy option. Since China has one of the world’s lowest water allocations per capita, it’s possibly just a matter of time before China begins to transfer water out of the Mekong and deliver away from Southeast Asia to its eastern urban zones.

Lastly, the Himalayan glaciers are melting at a rate faster than most frozen places on earth. It is possible that China’s huge Mekong dam reservoirs are built to catch this runoff, saving rainfall and snowmelt for a time years from now when it’s needed for electricity production or even diversion into the Yangtze River system. 

China consistently holding back more wet season water

Between 1992 and 2019 China’s Mekong Dams restricted increasing amounts during the wet season – a time when water is critically needed downstream.

Eyes on Earth’s second major finding is that China’s Mekong dams restricted increasing amounts during the wet seasons and is releasing more in the dry seasons. In recent years, China has improved its notifications of releases and temporarily halted operations at Jinghong Dam, its most downstream dam. In 2002, downstream countries and the Mekong River Commission received notification of monsoon season releases from the Jinghong Dam as a precautionary measure to flood risk, and after 2010 China began sharing information on dry season releases. But despite these recent improvements, Eyes on Earth’s findings show that water releases increasing during the dry season and water restrictions increasing during the wet season. This effect is particularly emphasized over the last three years.

Knowing when water is restricted is just as critical, if not more important, than knowing when water is released. This data could be utilized by downstream stakeholders to press for more upstream releases, particularly during times of drought. 

Of the last ten major droughts in the lower Mekong basin, eight have occurred since China’s first dam began construction: 1991-94; 1997-98; 2002; 2003-2005; 2008 – 2010; 2012 – 2013; 2015 – 2016; and 2019 – 2020. 

Prior to the current drought, 2016 saw the worst drought on record and devastated the Mekong Delta. More than 2 million Vietnamese and the majority of Vietnam’s rice production area was impacted by low water levels and severe saline intrusion in 2016, resulting in over $670 million of losses. In March 2016, China’s efforts to release water from upstream dams to relieve a drought were met with much fanfare, even in Vietnam. But those dry season releases came at the expense of previous wet season restrictions that would have benefited the downstream – instead the 2015 upstream restrictions set up conditions necessary for the 2016 dry season releases.

The Eyes on Earth data shows that China’s behavior changed in 2012, as the amount of water restricted significantly lowered flow compared to previous wet seasons in some years. This change coincides with completion of the Nuozhadu Dam which came online over a two year period between 2012 and 2014. Nuozhadu is the largest single dam in the cascade, with a storage capacity of about half of the Chesapeake Bay. After it came online, more water was held back during the monsoon season in particular, and in greater amounts.

Click to Zoom

The Eyes on Earth study also reveals more than a few instances when a massive reduction of flow in one year is then followed by a significantly higher than expected outflow the following dry season. For instance, in 2002 the model predicted significantly higher water levels during the monsoon season than the river gauge recorded, and in 2003 much more water was released than the model predicted during the dry season. The same pattern is observed in 2012 – 2013 and then in 2014. This coincides, and is likely related to, completion of some of the larger dams on the mainstream. Dachaoshan came online in 2002 and Nuozhadu in 2012. 

Eyes on Earth speculates the sudden and unexpected releases are an act of showmanship on behalf of dam operators where China’s elite can witness the incredible power of a fully stocked and operational dam. But rarely is the pattern repeated because the dams are only occasionally used after initially coming online. But each time these unseasonal and sudden releases occur, the downstream is hit with a tidal wave: the Chinese dam operations suck an enormous amount of water out of the system in one year, and then unleash it with reckless abandon on the downstream in the following year. In most occurrences, no warning is given to the downstream. The most egregious of these events happened in December 2013 when sudden upstream releases caused a 10-day flood in Thailand’s Chiang Rai province causing millions in damage with zero prior notification. 

Communities in Thailand’s Chiang Rai Province have long voiced desperation over how these unexpected rises in the river level, particularly during the dry season, inundate riverside farms and whisk away livestock and farming equipment stored on the riverside. These shocks also impact the life cycles of migratory fish and birds which depend on natural seasonal fluctuations for migration and nesting. Further downstream these irregular shocks contribute to changes in the Tonle Sap Lake where both the extreme high waters of the monsoons and the low waters of the dry season are needed to produce the lake’s annual expansion and contraction that generates a fish catch of more than 500,000 tons for Cambodia.

Click to Zoom.

Above: The unexpected release of water from China’s dam cascade caused sudden and severe flooding around Chiang Saen in the middle of the dry season. The flood resulted in millions in damage to local communities. Chinese authorities gave no downstream warning.  

Opportunity for Lower Mekong Policymakers

This data can help the Mekong River Commission engage with China

The Eyes on Earth study reveals the high level of control that China has over the water flow to downstream countries. If the collection and presentation of this data is regularized and shared openly, it could be utilized for near-time analysis and drought monitoring. Such a tool would be useful for a wide variety of stakeholders: government planners in the downstream Mekong countries; operators of the mainstream hydropower projects in Laos, such as the Xayaburi Dam, which also need predictable and sufficient flows of water efficiently operate; and most importantly, the riparian communities who are directly impacted by the changes to water flow wrought by China’s dams.

This study proves beyond reproach how deeply impactful China’s dams are for the lower Mekong. While the presence of China’s dams cannot be altered, China can and should change the way it operates these dams. This study provides an opportunity for downstream countries and the Mekong River Commission to collectively engage with China on ways to constructively manage water in the Mekong basin. Now that there is clarity on when and where water has been held back (or released), discussions with China can be based on evidence-based data and analysis rather than uncertainties and speculation.

Currently the MRC and China’s Lancang Mekong Cooperation Mechanism are jointly conducting an investigation to identify the root causes of the 2019 drought. The Eyes on Earth study provides clear evidence to lay to rest any doubt toward the role of China’s upstream Mekong dams. The Eyes on Earth study should also open a door to discussion of how water management can be improved moving forward. The 2019 – 2020 drought will not be the last severe drought which impacts the Mekong region. While China’s behavior in this instance greatly exacerbated drought conditions, transparency of information and more cooperative engagement could transform China’s dams into a solution for the next time a major drought hits the region.

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