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As hydropower dams quell the Mekong’s life force, what are the costs?

Fishers pull in nets on Tonle Sap Lake in Cambodia

Fishers pull in nets on Tonle Sap Lake in Cambodia where fish from the mainstream Mekong River are crucial for livelihoods and food security. Image by Carolyn Cowan/Mongabay.

  • The Mekong River is one of Asia’s longest and most influential waterways, sustaining extraordinary species and biodiverse ecosystems and providing nutrition for millions via its fertile floodplains and unparalleled fisheries.
  • But over the past few decades, the construction of hydropower dams has undermined the river’s capacity to support life: more than 160 dams operate throughout the Mekong Basin, including 13 on the river’s mainstream, with hundreds more either planned or under construction.
  • Besides severing fish migration routes and natural sediment transport throughout the river system, the dams affect the river’s natural seasonal ebb and flow, an ancient rhythm alongside which ecosystems have evolved.
  • Communities, scientists and decision-makers now face unprecedented challenges as fish catches dwindle, riverbanks erode, ecosystems collapse and the delta inexorably sinks.

This is the first article in a Mongabay series focused on changes to the ecology and hydrology of the Mekong River. Read Part Two.

The Mekong River carves a vast aquatic lifeline through Asia. Rising in glacial streams high in the Tibetan plateau, the river morphs as it tumbles south through rocky ravines, steep-sided valleys and expansive flooded forests to the South China Sea. Its influence is immense: over its 4,350-kilometer (2,700-mile) course, the Mekong drains a basin spanning hundreds of tributaries and six countries: China, Myanmar, Thailand, Laos, Cambodia and Vietnam. But the scale of its epic proportions is far outweighed by the richness and diversity of life it sustains through its biodiverse ecosystems, unparalleled fisheries and fertile floodplains.

Its vast biodiversity rivaled only by the Amazon, the Mekong’s muddy brown waters are a refuge for an array of extraordinary species, many of which occur nowhere else on the planet; from critically endangered Irrawaddy river dolphins (Orcaella brevirostris) and Asian giant softshell turtles (Pelochelys cantorii) to Mekong giant catfish (Pangasianodon gigas) and the world’s largest recorded freshwater fish, the giant freshwater stingray (Urogymnus polylepis).

With so much biodiversity, the river yields one-fifth of the world’s total freshwater fish catch every year. More than 1,000 types of freshwater fish migrate seasonally up and down the river, fueling wild fisheries that provide food security and livelihoods for tens of millions of people.

The Jinghong Dam.
The Jinghong Dam was built on the Lancang (Mekong) River in China. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

Floodplains in the Mekong’s intensively farmed delta in Vietnam are also a source of sustenance and stability. Sometimes referred to as Southeast Asia’s rice bowl, the delta is an agricultural powerhouse, producing fruits, vegetables and rice that feed the wider region and are exported to the world.

In short, the river shapes not only the landscape it passes through, but also the health of ecosystems, ways of life, and food and economic security of the whole region.

Yet colossal dams are being built to harness the Mekong’s power. To date, more than 160 hydropower dams operate on the river and its tributaries, including 13 on the river’s mainstream, with hundreds more either planned or under construction. Much of the energy generated powers local grids, but increasingly, deals are being struck to export power outside of the Mekong region to such countries as Malaysia and Singapore.

Debate has bubbled among the Mekong’s six countries for several decades on the harm the dams are doing to the river and those who rely on it. But a fresh urgency has emerged in recent years due to a suite of profound changes in the river system. Dwindling fish catches, unprecedented riverbank erosion, collapsing flooded forests and a sinking delta are just some of the challenges facing the Mekong’s riverside communities and decision-makers.

While many of these threats have surfaced since the completion of some of the largest dam projects, multiple other pressures complicate the picture. The region experienced a string of abnormally hot and dry years between 2019 and 2021, for instance, during which time the river ebbed to its lowest level on record. In addition to climate change, the effects of dams are further compounded by overfishing, sand mining and rampant deforestation up and down the river, muddying the waters when it comes to figuring out exactly what changes are due to dams and, crucially, what can be done to mitigate their effects.

Monitoring initiatives focused on the region are now documenting evidence of the ways hydropower dams impact the river. Findings show that dams are unequivocally affecting the driving force behind the river system’s impressive ecological richness: its wet season/dry season ebb and flow.

The Mekong River looking north toward Myanmar.
The Mekong River looking north toward Myanmar. Thailand is on the left and Laos on the right, where the Golden Triangle development zone dominates the landscape. Image by Carolyn Cowan/Mongabay.

An ancient rhythm in flux

The Mekong’s seasonal ebb and flow, often referred to as its “flood pulse,” is deeply connected to the region’s monsoonal climate. During the rainy season, typically from June to November, the whole river system transitions into a flood phase. Swollen by rainwater, the river inundates surrounding land, depositing nourishing sediment on floodplains and transforming forests into fish nurseries.

By the time the flood pulse reaches Phnom Penh in Cambodia, the water flow is so great that it reverses the flow of Tonle Sap River, channeling water north into Tonle Sap Lake, which can expand up to five times its dry-season size to cover an area half the size of Belgium. The fish nursery and spawning grounds in the great lake are so productive that fish caught in the lake provide the people of Cambodia with up to 70% of their dietary protein.

Conversely, the floodwaters recede with the arrival of the dry season in November, exposing newly fertilized floodplains and riverbanks on which migratory birds lay eggs and communities up and down the river grow crops, tend livestock and host events.

The Mekong’s ecosystems have evolved in step with this ancient seasonal rhythm. But now, hydropower dams are switching the tempo. Typically operated to optimize electricity generation, the dams require a steady year-round water flow. As a result, dam operators restrict water to build up reservoirs during the wet season, when the river should be in full flow, and release water during the dry season, when the river’s natural flow should be waning.

Flooded forests in Cambodia.
Flooded forests in Cambodia provide crucial fish nursery habitats but are being decimated in part due to changing water levels due to dams. Image by Carolyn Cowan/Mongabay.
Communities living on Tonle Sap Lake.
Many communities living on Tonle Sap Lake rely on healthy fish abundance. Image by Carolyn Cowan/Mongabay.

Since December 2020, the Mekong Dam Monitor, a collaborative initiative between the U.S.-based Stimson Center and satellite research consultancy Eyes on Earth, has gathered evidence on how hydropower dams are altering the natural flow of the river. Using near-real-time satellite data and information from water gauges along the river, the monitoring team tracks water levels in 45 of the Mekong’s largest dams and surface wetness relative to historic levels throughout the basin.

The first complete year of Mekong Dam Monitor data indicated that hydropower dams have “inexorably” altered the river’s natural flow, with large dams compounding water shortages in downstream parts of the basin during some of the worst drought years in recorded history between 2019 and 2021, according to an analysis released in 2022.

Brian Eyler, Southeast Asia program director at the Stimson Center and co-lead of the Mekong Dam Monitor, said the latest proliferation of “megadams” on the mainstream Mekong and its tributaries likely played a key role. “After 2018, there were a number of large dams that came online that could have exacerbated the [drought] situation in 2020, like the Lower Sesan 2 dam in Cambodia, like the Xayaburi dam in Laos, and three dams in China,” Eyler told Mongabay.

The 2022 analysis showed that during the delayed onset of monsoon rains in 2020, river levels in the lower basin hit an all-time low and Tonle Sap Lake remained at dry-season levels during the height of the wet season when it should have been in full flood. Notwithstanding the downstream drought, two of China’s largest dams, the Xiaowan and Nuozhadu, restricted a combined 20.1 billion cubic meters (5.31 trillion gallons) of wet-season flow during 2020, Eyler said. That’s similar to the volume they held in 2018, a particularly rainy year.

Nuozhadu Dam.
Nuozhadu Dam on the Lancang (Mekong) River in China. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

These hefty restrictions by upstream dams disproportionately affect downstream water flow during particularly dry rainy seasons, Eyler added, impacting the river even as far as Stung Treng in Cambodia, 2,500 km (1,550 mi) south of China’s southernmost dam at Jinghong. The Mekong Dam Monitor report also indicated that unseasonal water releases by upstream dams are causing abnormally high dry-season river levels, with Stung Treng one of the most severely impacted locations. Fish, birds and plants thrive in the flooded forests here, but riverside communities in the area report that since 2015, ancient trees have been dying out, likely due to year-round submersion.

The changes to the river’s seasonal flow are altering people’s lives across the basin, according to Gary Lee, Southeast Asia program director of International Rivers. “Large-scale dams are disrupting the flow of water and sediments, and damaging key aquatic ecosystems and resources, which are important for community livelihoods and food security,” Lee said.

“Usually, the water in the Mekong takes about three or four months to go from the lowest to the highest level,” Teerapong Pomun, director of Thailand-based NGO Living Rivers Association, told Mongabay. “But now, even on a daily basis, the water level changes by up to 1 meter [3 feet] and that impacts fish migration, fish populations and ecosystems.”

Teerapong added that many riverside communities can no longer depend on the river’s seasonal flow cycle to guarantee reliable crop harvests and fish catches. “The Mekong ecosystem and livelihoods have changed so much,” he said, “many villagers have lost their hope with the river.”

Fish during the migration season
Fish during the migration season. The river stretch between northern Cambodia and southern Laos is an important fish migration corridor. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

A glut of dams

Despite mounting evidence of problems, there are no signs of dam developers losing interest in large-scale projects. According to the Stimson Center’s Mekong Infrastructure Tracker, some 34 hydropower projects are under construction in the Mekong Basin as of late March 2023.

To date, China operates a cascade of 11 megadams on its stretch of the mainstream river, known locally as the Lancang. This has essentially transformed the once free-flowing upper basin into “a series of stepped reservoirs,” according to Philip Hirsch, a Mekong specialist and emeritus professor of human geography at the University of Sydney. A further eight dams are either planned or under construction on the mainstream river within China’s borders.

Several of the world’s largest storage reservoirs are stacked in the China cascade, including the Xiaowan and Nuozhadu projects, which together hold 38 billion m3 (10 trillion gal) of water, representing half of the total active storage volume throughout the entire Mekong Basin.

As part of its aim to become the “battery of Southeast Asia,” Laos has two dams operating on the Mekong mainstream and seven more scheduled for construction, including a highly contested 1,460-megawatt project situated 25 km (15.5 mi) upstream of Luang Prabang, a UNESCO World Heritage Site in the north of the country. Laos is also pressing ahead with a controversial 86 MW project sited on the Sekong River, an important tributary of the Mekong that represents one of the few remaining fish migration routes in the river basin.

Of Laos’s two mainstream dams, the 1,285 MW Xayaburi project has received criticism from downstream communities in Thailand who reported reduced fish catches and lack of sediment in the river following the dam’s completion in 2019. Further downstream, the government of Cambodia announced in 2020 a 10-year postponement of plans to build two mainstream dams at Sambor and Stung Treng, a move that observers say could have been motivated by concerns over the well-known ecological risks to Tonle Sap Lake and its vital fisheries. However, in early 2022, feasibility studies were completed to scope out the 1,400 MW Stung Treng project, raising fears and concerns among locals and experts that the project has been resurrected, despite the government’s declaration.

Fish processing on Tonle Sap Lake, Cambodia.
Fish processing on Tonle Sap Lake, Cambodia. Image by Carolyn Cowan/Mongabay.
Reservoir of Xiaowan.
Reservoir of Xiaowan. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

Can cooperation spark real change?

With so many profound changes to the river system, questions are being asked of the Mekong River Commission (MRC), the intergovernmental body responsible for transboundary water governance on the river. Established in 1995, the MRC works with the four lower basin countries of Cambodia, Laos, Thailand and Vietnam, with China and Myanmar as dialogue partners.

A report released by the MRC in early 2022 that investigated the underlying causes of the river’s recent low wet-season flow concluded that hydropower dams compounded the exceedingly low rainfall from 2019 to 2021 and fundamentally altered the river’s natural hydrology.

However, despite its own evidence of the threats posed by hydropower dams, the MRC has no regulatory powers to prevent the construction of new projects along the river, leading observers to criticize the body as ineffective in shaping a regional development agenda that holds the river’s health in mind. While some see the MRC as merely overseeing the river’s demise, others point to its focus on technical reporting and procedures that fail to embrace the human and social impacts of damming the river.

Eyler said the “easiest physical way” to reintroduce natural flow to the river would be through negotiations with China on the timing of water restrictions and releases from its 11-dam cascade. But that would necessitate a firmer stance from the MRC in its dialogue with China, he said, citing a May 2022 MRC report that extolled the virtues of China’s dry-season water releases.

River rapids in southern Laos.
River rapids in southern Laos. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

“The MRC have got to decide what their priority is,” Eyler said. “Do they fight for natural flow and ask [dam operators in China] for more wet season releases and a return of natural flow to the system? Or do they continue to praise China for releasing water during the dry season, [a process] that is ecologically damaging to the system and comes with the trade-off of wet season restrictions?”

China’s hydropower development plans and operations have been routinely opaque. The country only recently agreed to share year-round water level data from two monitoring stations on the Lancang with downstream countries. The unprecedented move was facilitated through the China-led Lancang Mekong Cooperation (LMC) mechanism, a development initiative launched in 2016 to promote policy dialogue between China and the lower basin countries.

Although the newly available data from China is “a big boon” for understanding how the river will behave immediately below China’s cascade of dams, Eyler noted that information on how dams are operated in China, such as the timing and volumes of water releases and impoundments, remains unshared.

Nonetheless, using the new water level data from China, the Mekong Dam Monitor has developed an online warning system to notify downstream communities of significant water restrictions and releases. It issues a “hydropeaking alert” whenever downstream water levels are expected to fluctuate by 50 centimeters (20 inches) or more within any 24-hour period, giving people time to move boats, livestock and farming equipment out of harm’s way.

The improved levels of data sharing are broadly welcomed among experts and riverside communities, but many agree with Eyler that a lack of operational data from dams throughout the basin stymies progress toward solutions.

An Irrawaddy river dolphin.
An Irrawaddy river dolphin. The only population of this species in the Mekong now resides in northern Cambodia. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

“Lack of transparency and accountability is pervasive in the development and operation of large-scale hydropower in the Mekong region,” said Lee of International Rivers. In addition to a dearth of operational data, he said, very little information on the efficacy of social and environmental mitigation measures is made public once projects become operational.

Most barriers to transparency arise out of the fact that the majority of hydropower projects are run as private ventures, according to Lee. Dam operators frequently cite “business confidentiality” as the reason why they can’t share this type of information. As a result, the public remains in the dark about how and why dams are operating on the river, and researchers have difficulty assessing the potential for adjustments to dam operations that might help downstream ecosystems and communities cope with the rapidly changing river system.

Given ongoing construction work within the basin, the reality is that new dams are inevitable. As each new project is completed and commissioned, the river’s connectivity for migrating fish and transport of vital sediments that sustain riverbanks and the delta will diminish. And, crucially, with every dam, the river’s lifegiving flood pulse will be reined in a little bit more. But whether those with the power to make decisions can maintain the Mekong as a living river and regional provider remains to be seen.

“The spirit of cooperation in the Mekong is deepening,” Eyler said. “But is that cooperative spirit translating to the urgency that is needed?”

Banner image: Fishers pull in nets on Tonle Sap Lake in Cambodia where fish from the mainstream Mekong River are crucial for livelihoods and food security. Image by Carolyn Cowan/Mongabay.

Carolyn Cowan is a staff writer for Mongabay. Follow her on Twitter @CarolynCowan11

Read Part Two:

Robust river governance key to restoring Mekong River vitality in face of dams

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