Considered the world’s most productive inland fishery, the seasonal flooding of Tonle Sap Lake — which can expand up to five times its dry-season width — has long been welcomed by the millions of Cambodians living on its shores.
But the country’s largest lake and fish basket is struggling to maintain its natural flood pulse due to a combination of human-made threats.
Irregular weather patterns attributed to climate change and a fleet of upstream dams along the Mekong River and its tributaries have disrupted the Tonle Sap’s ability to properly expand during the wet season, a phenomenon critical to aquaculture and food security.
While the lake’s water levels were close to normal in July, they fell significantly in August. These fluctuations align with a long-term recession of the Mekong River system’s water levels over the last decade.
As downstream communities around the Tonle Sap and other tributaries adjust to the new norms of unpredictable flood-pulses, the primary suspects in this crisis are the hundreds of dams in the Mekong Basin and a regulatory body with limited cooperation from its members.
Expanding Dams, Shrinking Flood Pulse
“The peak of the [Tonle Sap] annual flood pulse is not reaching its normal height at the time that it should – and rarely does it reach its normal height at all during the course of the wet season,” said Brian Eyler, who directs the Stimson Center’s Southeast Asia and Energy, Water and Sustainability programs.
According to data collected by the Mekong River Commision, the Tonle Sap river – the tributary which reverses its flow direction during wet season, moving northward from its junction with the Mekong River then feeding into the Tonle Sap Lake – is around two meters below its average water level for this time of year.
However, Eyler noted that in July – the middle of wet season – water levels at Tonle Sap may have been normal without the impact of dams.
“But in August – and into September – the trend is downward, when the average line is still moving up. So you have the opposite trend of what should be happening, and a lot of that is due to dam impacts instead of precipitation related,” he said.
Considered the beginning of the “mega-dam era”, the onslaught of 2010 saw the development of dozens of major dam projects along the Mekong River and its arterial waterways. Many of these dams, spurred by energy demands from economic development, were built by China in the upper reaches of the Mekong, known locally as the Lancang River.
The impact of these mostly hydroelectric and water supply dams can be found throughout the Mekong Basin.
As for the Tonle Sap, upstream water retention from predominantly Chinese and Loatian dams significantly disrupts the natural flood pulse, reducing the crucial seasonal inflows that local communities rely on for their livelihood. The lower seasonal floods also produce knock-on effects for Cambodia’s national food security and the agriculture communities further downstream in Vietnam’s Mekong delta region, according to Eyler.
Reduced expansion of the lake limits fish access to shallow waters, which are essential for their feeding and growth. Additionally, decreased flooding results in less sediment and organic material, such as leaves and twigs, being washed into the lake. This further diminishes the food supply for fish and disrupts the ecology of both the lake and its shoreline, Eyler elaborated.
A decline in fish stocks will likely force fishing communities to find alternative livelihoods, a challenge for which Cambodia currently lacks a clear path forward, he added.
But it’s not just communities around Tonle Sap whose livelihoods have been disrupted by lower flood pulses.
Uncertain Livelihoods
Lim Sai, a 54-year-old fisherman in Stung Treng province, which borders Laos and is bisected by the Mekong River, said that peak water levels have not been seen in years.
“The biggest problem is the dry months. The [Mekong River] water levels are not stable; sometimes when there is more water, they open [the dams], and when there is less water, they close them, making it difficult to fish,” he said.
Sai speculated that the perceived decline in fish populations is primarily due to the impact of upstream dams. However, he also believes that the increase of illegal “electrofishing”, driven by reduced fish numbers caused by these dams, has further exacerbated the problem.
In Sai’s village, like many others, fishing has been banned from the beginning of June until the end of September to preserve the fish populations. During the off-season, he farms to sustain himself.
Im Rachna, spokesperson for the Ministry of Agriculture, Forestry, and Fisheries, said the ministry is sympathetic to fishermen who say their catch is smaller than previous years.
However, Rachna maintains that any lower than normal fish yields during this season are more likely due to overexploitation, rather than the reduced flood pulses attributed to dams and climate change which prevent fish from circulating inland.
“In fact, fish stocks in 2023 increased by 5 percent compared to 2022, not decreased,” she said, referring to all of Cambodia’s fisheries.
According to the Fisheries Administration, 426,750 metric tons of freshwater fish were caught across Cambodia in 2023. Although this was higher than the yields in 2019, 2021, and 2022, the three-year stretch from 2019 to 2021 marked the driest years on record for the Tonle Sap, where a low flood pulse limited the output of Cambodia’s most productive fishery.
Discrepancies in Dam Tracking
While riparian communities continue to speak out about the impacts of dam projects and climate change, the Mekong River Commission (MRC)—the intergovernmental organization responsible for coordinating the sustainable management of the Mekong Basin—has struggled to address these impacts and obtain a clear count of the dams to track.
The MRC estimates there are 61 water diversion projects along the Mekong and its tributaries.
Of the four signatories of the 1995 Mekong Agreement, which established a framework for regional water cooperation and led to the creation of the MRC, Cambodia has the fewest dams, according to the commission.
Thailand, Laos, and Vietnam are the other countries who ratified the agreement and became members of the MRC. China, despite its significant impact on the river, has neither signed the agreement nor elevated its status from “dialogue partner” to full-member of the MRC.
Albeit, in 2016, the Lancang-Mekong Cooperation was established to prompt cooperation among China, members of the MRC and Myanmar for managing and developing the shared water resources of the Lancang and Mekong Rivers.
“We work closely with our member countries to monitor hydrological data and facilitate dialogue among member countries. We conduct impact assessment, promote joint monitoring and foster cooperation with upstream countries like China,” the MRC Secretariat said via email.
“This collaboration will involve data sharing, joint projects, and basin-wide initiatives aimed at addressing water security concerns, mitigating the impacts of floods and droughts, and promoting sustainable development,” the commission added.
While MRC members and China commit to share-real time data and updates their dam activities, the Stimson Center and other regional groups believe most countries are not fully disclosing this information.
In large disparity with the commission’s reporting, the Stimson Center has registered a
total of 745 dams operating or under construction on the mainstream and tributaries of the Mekong Basin, 40 of which are in Cambodia.
Geoscientists from various universities in the region and abroad published a report in early 2024, stating there are 1,055 dams.
“Our team knows, and most other Mekong watchers know that there are now hundreds of dams in the Mekong Basin, including those on tributaries,” said Eyler. “So that [the report of 61 dams] says these countries are not notifying the MRC when new tributary dams are being built.”
Although the MRC engaged with CamboJA News, it did not address a question about the reasons for disparities in dam tracking data.
Mar Sophal, program manager at the NGO Forum on Cambodia and who has worked with communities affected by dam projects, suggested the member states of the MRC should continue to seek international consultation and request further cooperation with China in response to the ongoing problems caused by their development projects.
However, the demand for increased Chinese cooperation arises just as Cambodia broke ground on its own development project—the USD $1.7 billion Funan Techo Canal—which will divert water from the Mekong River, with minimal international input beforehand.
While the environmental consequences of this project are not yet fully understood, Eyler believes Cambodia has lost its former status as the biggest champion of the Mekong Agreement due to the development.
Be that as it may, if regional consultation occurs and effective mitigation efforts are implemented, the canal could benefit all involved countries, enhance Cambodia’s economic development, and strengthen adherence to the agreement’s protocols for future projects. This would help prevent a negative cycle and set a positive precedent for other developments in the region which ultimately affect the water levels at the Tonle Sap, Eyler noted.
