Stung Treng, Cambodia — Giant acacia trees, greyed by death, stand submerged in the flooded forest around the Mekong River in northern Cambodia. It looks like a cyclone has torn through the area. But the cause of the trees’ demise is more insidious. For years, upstream dam operators, motivated by economic gains, have released too much water into the river during the dry season, leaving levels artificially high when the trees need to dry out, causing many to die.
The slow decline of this flooded forest, which connects to the river here and extends into neighboring Laos, is alarming. The forest plays a vital ecological role in what is an essential stretch of the world’s most productive river for fisheries. “This is the most important spawning ground for fish in the Mekong,” says Srey Somuichet, who directs the regional fisheries administration in Stung Treng, Cambodia.
The forest isn’t the only wetland area hurting along the Mekong River, which runs for 2,700 miles through Southeast Asia. The river’s lower basin contains more than 70,000 square miles of wetlands, an area larger than Florida. But since 1970, 30 percent of Mekong wetlands have been lost to agricultural encroachment, hydrological changes from infrastructure projects, and other threats.
Despite this, conservation efforts in the Mekong region have mostly focused on the main river and its many tributaries. Tens of millions of people rely on the river for agriculture and fishing, and researchers are recognizing the critical role that wetlands play in the ecosystems that support these industries. New developments, such as a controversial canal project in Cambodia, also add urgency to wetlands preservation efforts.
“If the water of the Mekong is the blood, the wetlands are its beating heart,” says Jake Brunner, head of the Lower Mekong program for the International Union for Conservation of Nature (IUCN). The organization is spearheading a forthcoming project worth nearly $100 million aimed at saving wetlands in the region. (Read our past coverage of threats to the Mekong River.)
What’s lost with wetlands
Wetlands—defined as ecosystems flooded or saturated by water, including floodplains, marshes, and mangroves—are vital ecological guardians worldwide. They support biodiversity, act as natural water filters, and buffer against floods. In the Mekong Basin, the stark difference between dry and wet season river flows magnifies wetlands’ importance. During the rainy season, powerful flooding injects life into vast floodplain habitats. These natural, seasonal fluctuations are called the flood pulse, and they’re essential for fish and vital for nourishing agriculture.
But those habitats are often understudied. While the stretch of river in northern Cambodia has been designated a Ramsar site—a wetlands area of international importance—little research has assessed the ecological damage to the flooded forest there. “What we have is basically interviews with people,” says Ian Baird, a geographer at the University of Wisconsin-Madison, who has studied fisheries across the border in Laos for several decades.(Learn about how megadams threaten the Mekong in Laos.)
Cambodian fishers described dry season water levels up to two meters higher than historical norms, Baird reported last year. He attributes the change to dam construction that began upstream in China and Laos in the 1990s. Dams store water during the rainy season and release it gradually during the dry season to keep producing energy.
Baird also found that up to 50 percent of two large tree species have been lost, and some bushes have experienced an almost 100 percent decline. Some fish species rely on such plants for food, as well as for nursing and spawning habitats, and plant losses negatively impacted fish populations, including commercially important catfishes.
Vegetation loss has also altered the physical river structure. Trees and bushes reduce the force of the river’s flow, allowing sandbars to build up. Fewer trees mean fewer sandbars, leading to less habitat for animals, including birds. “We know we have a lot to lose, that’s not in doubt,” says Baird.
More floods, more fish
Ecologically, the most crucial wetland area in the Mekong Basin is Southeast Asia’s largest lake, Cambodia’s Tonle Sap, which connects to the Mekong via the Tonle Sap River. This lake is surrounded by forests that flood during the rainy season. Floods expand the lake many times beyond its normal size and provide vital feeding and nursery habitats for hundreds of fish species.
Up to 400,000 tons of fish may be caught in the lake every year, though reliable figures are scarce. In comparison, the annual fish harvest from all U.S. rivers, lakes, and reservoirs combined is about half of that amount on average.
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But Tonle Sap fish catches have dropped significantly in the last 20 years due to overfishing and profound changes in the lake. The flooded forests have lost more than 30 percent of their area since the 1990s, mainly due to agricultural expansion and fires. Droughts in 2019 and 2020, combined with dams withholding water upstream, led to a significant slowdown of the flood pulse that brings Tonle Sap to life, with catastrophic fallout for fish populations.
“The magnitude of the flood pulse is directly related to the amount of fish the system produces,” says Zeb Hogan, a biologist at the University of Nevada, Reno, who leads the USAID-funded Wonders of the Mekong research project. “Both fish and forests are adapted to how the flood pulse produces seasonal highs and lows.”
In recent years, lake flooding has increased to more normal levels. Simultaneously, the Cambodian government has clamped down on illegal fishing and curtailed agricultural expansion. As a result, small but hopeful signs hint that the fisheries have rebounded. For example, one Tonle Sap fishing operation in flooded forests near the city of Siem Reap, monitored by Hogan’s team, has reported increased catches for the last three years. (Read more about efforts to save freshwater fish in Tonle Sap.)
“It’s been clearly demonstrated that more inundated wetlands equals more fish,” says Hogan, who is also a National Geographic Explorer.
Even giant species show up. On a recent visit to wetland areas in southern Cambodia, Hogan was surprised to learn that fishers had caught a 400-pound Mekong giant catfish in seasonally inundated wetlands. “It’s wild to think that an animal like that would be cruising around in an area that is completely dry for much of the year,” he says. (See photos of the world’s largest freshwater fish.)
Putting a price tag on wetlands
The proposed Cambodian-Chinese Funan Techo canal project could threaten wetland areas in southern Cambodia and Vietnam. This new canal would connect Phnom Penh with Cambodian ports in the Gulf of Thailand, bypassing Vietnam’s control over the Mekong’s mouth.
Set to begin operating in 2028, the project will convert smaller, low-levee canals into a 300-foot-wide, two-lane high-levee canal. Brian Eyler, director of the Southeast Asia program at the Stimson Center in Washington, DC, warns that the canal could alter water flows and disrupt natural flooding in the Mekong Delta, Vietnam’s agricultural and economic engine.
Cambodia has not released environmental studies for the project but asserts it does not draw water from the main river stem of the Mekong. This prevents the regional Mekong River Commission (MRC), which promotes the sustainable development of the basin, from getting involved, as its mandate only covers the Mekong River itself.
Observers point to the limited mandate as an example of how wetlands and other ecosystems are often ignored in decision-making. “Planners and engineers see rivers in straight lines with a beginning and end point,” says Eyler.
Conservationists hope the current is changing for wetlands with projects like the upcoming wetlands-based adaptation initiative led by the IUCN with financial support from the French government. The initiative aims to restore and better manage the region’s most vulnerable wetlands, including the Mekong Delta, Tonle Sap Lake, and the flooded forest in northern Cambodia. The project also seeks to assign financial values to the ecosystem services these wetlands provide.
Rafael Schmitt, a research engineer at Stanford University, emphasizes the importance of valuing wetland benefits. “If we do not capture wetland benefits in a holistic manner, we risk undervaluing these wetland ecosystems, which might skew decision-making,” he says.
In the flooded forest in northern Cambodia, satellite data offer a glimmer of hope: Dry-season water levels have decreased in the last two years, though they remain above historical norms.
Chamnan Hong, who directs the wetlands conservation department at Cambodia’s Ministry of Environment, hopes increased research engagement will help protect the flooded forest. “To save the Mekong, we must pay greater attention to the whole river system, and that means starting with the wetlands,” he says.
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