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Hydropower in doubt as climate impacts Mekong Basin water availability

Mekong River

The Mekong River flows between northern Thailand, Laos and Myanmar at the meeting point of the three countries, known locally as the “golden triangle.” Image by Carolyn Cowan/Mongabay.

  • Warmer and drier wet seasons in the upper basin of the Mekong River are affecting the availability of water for hydropower generation along the major watercourse, according to a new analysis.
  • At a recent online discussion, regional experts questioned the viability of hydropower on the Mekong as a long-term, sustainable energy solution, given the increasing presence of climate risks.
  • With large-scale dams in upper parts of the basin failing to fill their reservoirs, panelists at the event asked whether they are truly worth their documented impacts on downstream ecosystems, livelihoods and communities.
  • Panelists recommended continued information sharing and improved coordination of dam operations to preserve the river’s crucial flood pulse that triggers the seasonal expansion of Tonle Sap Lake in Cambodia, and also highlighted the conservation importance of the Tonle Sap watershed, including tributaries in Cambodia’s Cardamom Mountains.

Shifting monsoonal climate patterns that affect the availability of water in the Mekong River Basin are raising questions among experts about the sustainability of hydropower as a long-term energy solution in the region.

This was a major point of discussion at a recent online dialogue, hosted by the U.S.-based Stimson Center, in which regional experts reviewed the findings of hydrological monitoring carried out by the Mekong Dam Monitor over the course of the 2023 wet season, and looked ahead to what might unfold during the upcoming dry season.

The Mekong Dam Monitor, a collaborative project between the Stimson Center and satellite research consultancy Eyes on Earth, tracks 55 of the largest dams throughout the Mekong and gathers hydrological information about the river basin using satellite imagery, data from water gauges, and surface wetness trends.

Panelists at the Jan. 26 event analyzed the complex interactions between dams and climate, questioning whether hydropower presents an economically viable long-term energy solution given that increasingly dry wet seasons are likely to compromise the productivity of dams in the river basin. They also proposed recommendations for improved management of the pivotal watercourse that traverses six countries.

Scores of hydropower dams have been built in recent decades to harness the river’s flow for energy production, including 13 major projects that span across the river’s entire mainstream channel, with hundreds more either planned or under construction.

Dams holding back water during the wet-season months to refill reservoirs have exacerbated severe droughts experienced by people and ecosystems downstream; and water releases to generate power have at times doubled the amount of water during certain dry-season months, the Mekong Dam Monitor reports in its 2022-2023 annual report.

The ensuing disruption of the river system’s natural seasonal flow has been shown to profoundly affect downstream ecosystems, livelihoods and communities. In particular, dam operations rein in the river’s seasonal “flood pulse” that drives key processes, such as the timing of fish migrations and the annual expansion of Cambodia’s Tonle Sap Lake.

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.

Climate factors cast doubt over dams

Brian Eyler, Southeast Asia program director at the Stimson Center and co-lead of the Mekong Dam Monitor, said dams in China have made some significant discharges of water so far in the 2024 dry season, citing the release of nearly 1 billion cubic meters (264 billion gallons) of water from China’s Xiaowan Dam between Jan. 23 and 26, which was expected to raise river levels at Chiang Saen in northern Thailand by more than 1 meter (3 feet).

Such unseasonable raising of water levels during the dry season, which typically runs from December to March, imposes negative consequences downstream, Eyler said. “At this point in the year, the river should be going down,” he said. In addition to sending confusing biological cues to migratory fish, “higher [water] levels really create a mess for the people of the Mekong during the dry season.”

Alan Bassist, president of Eyes on Earth and co-lead of the Mekong Dam Monitor, said the 2023 wet season saw climatic warming conditions in the Mekong’s upper basin, including in the snowpack of the Tibetan Plateau where the Mekong River rises. “Temperatures are much warmer than the long-term average,” Bassist said, “and, of course, that’s impacting rainfall, river discharge and growing conditions.”

Eyler pointed to the warm and dry conditions in the upper basin to explain why 2023 saw less refilling of major dams in China, the largest two of which hold roughly half of the Mekong Basin’s total useable water capacity. China’s Nuozhadu Dam, for instance, filled to just 50% of its total storage volume in 2023, according to the Mekong Dam Monitor data, and has only ever filled fully during half of its operational years to date.

For Eyler, the struggle to fill leviathan dams raises serious questions about development pathways in the Mekong Basin. “Is it worth having these huge dams on the Mekong if they’re not doing what they’re supposed to do?” he asked.

Reservoir of Xiaowan.
Xiaowan Dam has the second largest storage reservoir in the Mekong Basin, spanning the mainstream river, locally named the Lancang, in China. Image by International Rivers via Flickr (CC BY-NC-SA 2.0).

When large dams like Nuozhadu fill to less than capacity, it translates to less impactful environmental outcomes downstream — “a good thing overall for the Mekong, for communities, for fisheries, for local agriculture,” Eyler said. “But if you’re an investor in the dam, it’s not making the money that it’s supposed to be making, it’s not generating the power that it’s supposed to be making.”

Eyler also underscored that data from water gauges along the Mekong clearly indicated that wet-season water levels during 2023 were consistently lower than long-term averages, even taking climate factors into account. He said such water-level anomalies effectively show that the upstream dams were still impacting downstream river flow, despite the fact they were holding relatively less water.

Climate-related risks to hydropower are well-recognized across the wider region. A 2021 study by the International Energy Agency, for instance, found that South and Southeast Asia’s hydropower capacity is likely to drop by roughly 4.6% by 2059 due to rising temperatures, extreme rainfall patterns, melting glaciers, and an increasing occurrence of extreme weather events associated with climate change.

Since the majority of hydropower dams on the Mekong were designed to operate within long-term climate averages, Bassist urged energy planners to take shifting climate patterns into account, citing alternative renewable energy sources, such as wind and solar, which could offer more stable pathways to future regional electricity production.

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.

Nourishing waters

Another key concern is the seasonal expansion of Cambodia’s Tonle Sap Lake, which depends on a wet season “flood pulse” of water flowing from the mainstream Mekong and reversing the flow of the Tonle Sap River. The flood pulse is essentially what drives the fertility of the Tonle Sap Lake, Bassist said. “It brings in the [fish] spawn, it brings in the larvae … which the whole Khmer culture is dependent upon, for the fisheries, for the food security and for the cultural traditions.”

Data collected by the Mekong River Commission indicates that the reversal of Tonle Sap River is increasingly occurring later in the year, delaying the onset of the vital lake’s expansion. In 2023, the reversal, which normally happens in July, began in earnest in August and only attained long-term average levels by September and October. In 2022, the reverse flow was weaker still, the lowest recorded in 30 years of recordkeeping.

“[For] millions [of people], their livelihood is dependent upon the fertility and the bounty of the lake,” Bassist said. “And it’s just not coming in to the levels that it normally had, and part of it is climate and part of it is the impacts of the reservoirs upstream.”

Even though the Mekong’s flood pulse only produced a “nominal” reverse flow into Tonle Sap Lake in 2022, the lake did expand to long-term average levels that year, the team reported. This was down to water reaching the lake basin from tributaries in the Tonle Sap watershed besides the Mekong, such as rivers flowing from Cambodia’s Cardamom Mountains.

“This highlights a need to prioritize the conservation of the Tonle Sap Watershed,” the Mekong Dam Monitor report says. “The Tonle Sap’s tributaries are both understudied and being exploited at an alarming rate by the construction of small and medium-sized dams and irrigation canals.”

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).

But crucially, although the Tonle Sap Lake expansion can be driven by other means, by far the “healthiest” mode of expansion is from the Mekong’s nourishing flood pulse, Bassist said, citing the stronger flood pulse in 2023, which gave the lake’s fisheries a boost and resulted in the discovery of threatened fish species that hadn’t been caught there in many years.

Out of concern for the delayed and weakened onset of the Tonle Sap reversal, the panelists urged upstream dam operators to wait until later in the season before refilling reservoirs. They recommended shifting upstream water restrictions from July and August toward the typically wetter months of September and October to afford the flood pulse a chance to trigger the critical reversal of Tonle Sap River. “If the water’s there during the later months then perhaps dams can wait to do their restrictions, and give the Tonle Sap and the system a chance to initiate,” Eyler said.

In response to a question from an audience member at the online discussion about what can be learned from approaches to other large river systems around the world, Bassist cited the case of the Columbia River Basin of the Pacific Northwest region of North America, where efforts to remove dams are now underway in recognition of the harm to the welfare of impacted communities and economic resources, such as salmon fisheries.

“Using knowledge from other experiences is always useful,” Bassist said. “What people have learned in different parts of the world that are parallel to what’s happening in the Mekong provides insights [and] could benefit better planning and policy around the Mekong Basin.”

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

Banner image: The Mekong River flows between northern Thailand, Laos and Myanmar at the meeting point of the three countries, known locally as the “golden triangle.” Image by Carolyn Cowan/Mongabay.

See related story:

Dam-building on Mekong poses risk to regional industries, report says

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