Because of a combination of threats from dams and climate change, the Tonle Sap Lake is facing the eminent risk of ecological collapse.  Once plentiful fish and other species are fast disappearing, along with the livelihood of the people which for centuries have depended on the Tonle Sap for subsistence.

“The beating heart of Mekong is now on life support”, said Brian Eyler, senior fellow and director of Energy, Water, and Sustainability, Southeast Asia at the Stimson Center.  He was referring to Tonle Sap, a lake in the northwest of Cambodia, which is part of the Mekong River system.

The Tonle Sap, the largest freshwater lake in Southeast Asia, can swell across a floodplain up to 20 times the size of Singapore during the monsoon season. 

According to journalist Abby Seiff, the lake, which awed ancient Chinese emissaries and European explorers with its vast size and bottomless fish population, is now at risk of ecological failure.

Threat from hydropower dams

China started damming the Lancang, its section of upstream Mekong River, in 1986.  Since then, China has built 11 hydropower dams along upstream Mekong within China’s borders and another 11 in Laos and Cambodia on the mainstream, along with hundreds of dams on its tributaries.

Hydropower dams have had a drastic effect on the Mekong River and its tributaries such as unseasonable flooding and droughts, low water levels in the dry season, and drops in the amounts of sediment carried by the river, with extreme consequences for biodiversity. 

Every year, the Mekong floods force the Tonle Sap to swell five times from its low-water size and thus creating the largest lake in Southeast Asia. However, in 2019, the lake was in a state of record low-level, the water was warm, shallow and oxygen starved.

Using satellite imagery and river height gauges, a study by Eyes on Earth, a water resources monitor, gave credence to suspicions that Lancang water policies were partly responsible for the record droughts.

“The satellite data does not lie, and there was plenty of water in the Tibetan Plateau, even as countries like Cambodia and Thailand were under extreme duress,” Alan Basist, who co-wrote the report for Eyes on Earth, told New York Times. “There was just a huge volume of water that was being held back in China.”

According to the report, during the 2019 drought, China’s portion of the Upper Mekong received uncommonly high levels of rain but the resulting flow was stopped by the Lancang dams.

The lack of cooperation over dam operations between countries combined with shorter monsoon season and a longer dry season because of climate change, have resulted in loss of habitats, making it difficult for bird and fish species to survive in the ecosystem.

Farmers have difficulty farming

Agriculture was once the backbone of Cambodia’s economy, however, the agriculture industry’s share of GDP had dropped from 47 percent in 1995 to 22.8 percent in 2021.

“Both infrastructure projects in the stream and climate change contribute to the low water level(s),” said Khoy Rada, a research consultant specializing in agricultural development at Angkor Research and Consulting. This has resulted in lower groundwater levels and higher evaporation of existing water resources. Communities along the Mekong, specifically those that rely on the Tonle Sap, are all set to be affected, Rada said.

Yin Savuth, the director of the Department of Hydrology and River Works at Cambodia’s Ministry of Water Resources and Meteorology, also acknowledged climate change consequences, dams as well as the less intense rainfall, caused the record low levels of water in Tonle Sap.

Hydropower dams have drastically altered the natural rhythms of the region’s water resources. The natural cycle of the Mekong floods reversing the flow of Tonle Sap, thus creating a lake, has come later for the past two years, devastating fisheries and complicating agriculture.

The dwindling water resources have been changing communities.  Many started to move closer to the lakes while others fought each other for water as the streams began to run dry. Farmers further from the lake, realizing that floods may never come again, are forced to trap water as it comes into Tonle Sap from its tributaries.

“Folks at the periphery of the Tonle Sap are super water-stressed,” said Brian Eyler. “The next village downstream (then has) their water stolen. Streams can be diverted and I’ve heard that it’s getting ugly in that peripheral band where this water-stealing phenomenon has taken off in the last decade or so.”

Fishermen that can’t fish

Tonle Sap was teeming with fish once upon a time, earning the title of the world’s most productive inland fishery.  The lake acted as a crucial nursery for more than 300 species of fish, whose larvae are carried downstream into the lake by the flood pulse.

However, in recent years, the combined threats of overfishing, illegal fishing, drought, habitat destruction and the impacts of upstream dams on the Mekong River’s natural flow have wreaked havoc on fish populations and aquatic habitats throughout the river system.

Fishing can no longer put food on the table.  This forced locals to move to urban areas for low-paying and notoriously difficult construction work, according to Oudom Ham, an independent Cambodian consultant on climate change and river issues.

“Now they cannot even catch fish for their own consumption. They need to buy, which is ridiculous because they are fishermen but they cannot find fish for themselves,” he said.

“Fishermen compensate for depleting fish catches by taking out loans to help them get by but fishermen can’t do this in perpetuity. Something is going to break, either the livelihoods of millions or regional food security,” said Eyler of the Stimson Center. 

The situation has been worsened by the COVID-19 pandemic.  Hou Savy, head of the Kampong Khleang Ecotourism Association in Siem Reap province, said that he has seen his community struggle through the Covid-19 pandemic.

“While Covid-19 has killed tourism and even the price of produce, the dams are killing the Tonle Sap. People used to be able to migrate to Thailand where there’s always work, but not now,” he said.

“No fish, no floods, no tourists and no migrant work – it’s really hard. I’ll sell my fish or my crops at low market prices,” he added. “The prices we’re selling at aren’t acceptable, but if there are buyers, we’ll sell. We all have debts to pay and they (the financial institutions) want their interest.”

“No one can definitively say whether the Tonle Sap has reached its ecological breaking point or when that breaking point will come,” says Brian Eyler. “We know it’s soon. But the thing that concerns me most is that it could be here and now.”

Photo credit: iStock/Sharon Lee

Jessica Loh

Jessica Loh

Jessica is a maritime student who is currently navigating her way through the industry.

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