Tonle Sap is the great lake of central Cambodia and the largest freshwater lake in Southeast Asia, joined to the Mekong at Phnom Penh by the Tonle Sap River, which reverses direction every rainy season. Its wetlands swell from about 2,500–3,000 km² and a depth of roughly 1 metre in the dry season to as much as 16,000 km² and up to 9–14 metres at the peak of the flood, drowning a ring of seasonally flooded forest, shrubland and rice fields. UNESCO designated it a biosphere reserve in 1997, Cambodia's Royal Decree of 2001 created the Tonle Sap Biosphere Reserve across nine provinces, and its core areas include three Ramsar sites: Boeng Tonle Chhmar (listed in 1999), Prek Toal and Stung Sen (2018). The protection is real on paper, but dams, forest clearance, fishing pressure and a changing climate are all wearing down the flood pulse that keeps the lake alive.
That pulse is the key to everything. From May or June the swollen Mekong pushes water up the Tonle Sap River at peak rates of about 10,000 m³ a second, and an average of 30 km³ flows backwards into the lake each year. In October or November the flow turns and the lake drains back out, an important supplement to the dry-season flow reaching the Mekong Delta in Vietnam. Fish spawn in the drowned forest, and roughly 1.2 million people around the shore live by catching them.
Lancang Dams, Xayaburi (2019) and Don Sahong (2020)
Because more than half of the lake's water comes from the Mekong, anything that changes the river's timing changes the lake. The first threat is therefore upstream. In China, where the Mekong is called the Lancang, the Manwan Dam became the first dam across the main river, and a cascade followed. As of February 2024, 15 hydropower plants stood on the Mekong mainstream, 13 of them in China and two in Laos. Even at Kratié in Cambodia, water from Yunnan makes up almost 30% of the average dry-season flow, so the way Chinese reservoirs are filled and released matters a long way downstream.
The 1995 Mekong Agreement put to the test
The lower basin countries, Cambodia, Laos, Thailand and Vietnam, coordinate through the Mekong River Commission, created under the 1995 Mekong Agreement and descended from the Mekong Committee set up in September 1957. China is not a member. The agreement requires a country planning a mainstream dam to notify the others and go through prior consultation, but the process is non-binding. When Laos proposed the Xayaburi Dam, the commission's Joint Committee announced on 19 April 2011 that the members could not reach consensus; Laos went ahead regardless, Cambodia's government protested, and the dam began commercial operation in October 2019.
The Don Sahong Dam, a 260 MW run-of-river plant in Laos less than 2 km upstream of the Cambodian border, followed the same path. Laos first argued it did not need prior consultation because it blocked only one channel of the river, agreed to the process in June 2014, and saw it end without agreement. Construction started in October 2015 and the plant was commissioned in 2020. Cambodian civil-society groups opposed it because of the risk to the fisheries that Cambodians, including the lake's fishing communities, depend on.
Lower Sesan 2 and the Funan Techo Canal
Cambodia has also built its own dams. The Lower Sesan 2 Dam on the Se San River in Stung Treng province, a 400 MW project with Chinese and Cambodian investors, produced its first power in November 2017 and was officially opened on 18 December 2018. Its environmental assessment found that at least 38,675 people in 86 villages would lose access to most of their fishery resources, and predicted effects as far away as the Tonle Sap. Downstream of the lake, the 180 km Funan Techo Canal, which had its groundbreaking on 5 August 2024, is planned to link Phnom Penh with the coast at Kep; Vietnam has raised environmental concerns, while Cambodia says it will draw only 5 m³ a second from the Mekong.
Flooded Forest, Fishing Lots and the Giant Catfish
The second set of threats is local. The freshwater swamp forest around the shore, the "flood forest" that covers only about 3% of the basin, is where fish feed and breed in the high-water months. Studies of forest loss find that the hotspots lie in the low floodplain, the same zone where the protected areas are, and that the main causes are population growth, fuelwood collection and logging, with the biggest expansion of farmland where the lower and upper floodplains meet. Hydropower development adds to the pressure by shrinking the natural buffers between farms and forest.
The lake is Cambodia's main fishery. It accounts for about 60% of the country's freshwater catch of over 400,000 tonnes a year, and fish from the Tonle Sap and the wider Mekong system supply an estimated 60 to 80% of Cambodians' protein intake, depending on the source. Many of the fishermen are ethnic Vietnamese living in floating villages, along with Cham communities; stilted villages such as Kampong Phluk near Siem Reap, home to 3,707 people in 2019, raise their houses 6 to 9 metres to ride out the flood. For more than a century the most productive parts of the lake were leased out as private fishing lots, bringing the government over US$2 million a year in tax. Even so, catches have been falling, illegal fishing and poaching persist inside the protected zones, and poverty around the shore makes overfishing hard to stop.
The lake holds at least 149 fish species, 11 of them globally endangered. The most famous is the Mekong giant catfish, which grows to about 3 metres; its numbers have fallen since the mid-1970s, and catching and keeping one is now illegal. Six were caught and recorded along the Tonle Sap River in December 2024. The Siamese crocodile, once found in the lake alongside the saltwater crocodile, survives in Cambodia at only about 30 sites with an estimated 200 to 400 animals. Invasive plants add a quieter threat: Mimosa pigra, a thorny shrub that Khmer speakers call "giant mimosa" or "Vietnamese thorn", is documented in both Cambodia and Vietnam and is a serious weed across Southeast Asia.
The 2001 Royal Decree and the Three Core Areas
The protection framework rests on UNESCO's 1997 designation and the Royal Decree of 2001, which made the lake and its floodplain the Tonle Sap Biosphere Reserve. It spans nine provinces: Banteay Meanchey, Battambang, Kampong Chhnang, Kampong Thom, Preah Vihear, Pursat, Siem Reap, Oddar Meanchey and Pailin. Like all biosphere reserves it is divided into a core zone, a buffer zone and a transition zone. Since 2003 a secretariat funded through the Asian Development Bank has tried to coordinate the farmers, fishers and agencies who all use the lake.
The core zone, about 42,300 hectares in all, is made up of three areas, each a Ramsar wetland. Prek Toal, in Battambang province at the lake's northwest corner, is a bird sanctuary whose colonies of large waterbirds are most crowded in the dry season. Boeng Tonle Chhmar, a 145.6 km² management area in Peam Bang commune, Kampong Thom, was established in 1999 and is Ramsar site no. 997; in the wet season its small lake disappears into the expanded Tonle Sap, and five floating villages lie around it. Stung Sen, near the lake's southeastern tip, has been a Ramsar site since 2018 and is a breeding ground for the sarus crane and lesser adjutant. Between them the core areas shelter spot-billed pelicans, painted storks, black-necked storks, Oriental darters, grey-headed fish eagles and greater adjutants, for which Tonle Sap is one of the few known breeding areas.
Designation has not stopped the damage. People still live and fish inside the core areas under Cambodian fishery law, and cutting flooded forest for farmland continues. The reserve's own planners have long admitted that balancing survival and conservation is the central problem.
Drought Years Since 2019 and the Weakening Flood Pulse
The newest pressure is climate. In July 2019 a stretch of the Mekong upstream, from Chiang Saen to Vientiane, fell to its lowest level in a century in what should have been the wet season. In July 2020, with an El Niño and dams on Mekong tributaries filling their reservoirs, Tonle Sap's water level hit a record low for that time of year for at least 60 years. UNICEF has noted that the dry season is getting longer and hotter while the rainy season is becoming harder to predict. A smaller reverse flow means less flooded forest, fewer fish larvae carried into the lake and less sediment, the source of the phosphorus that feeds its food chain; about 72% of the lake's modern sediment comes from the Mekong.
Models suggest that hydropower development could cause large changes in the lake's habitats by the 2030s. Against that, restoration is small but measurable: Conservation International Cambodia reported in 2024 that it had restored about 600 hectares of flooded forest and planted more than 270,000 seedlings since 2010, with a longer-term target set for 2030.
Tonle Sap cannot be protected from Cambodia alone, because its water, fish and sediment all depend on the river; follow that river in our complete guide to the Mekong, see where the lake's dry-season outflow ends up in our article on the Mekong Delta's wildlife, and set the lake in its national landscape with the geography of Cambodia.