At the end of the dry season the Tonlé Sap is barely a lake. It covers something like 2,500 to 3,000 square kilometres and runs about a metre deep — shallow enough that a person could wade an absurdly long way from shore, and shallow enough that boats stick to dredged channels. Then the reversal begins. By September or October the water has spread across as much as 16,000 square kilometres of floodplain, and depths of nine to fourteen metres cover ground that was rice stubble in April. The volume goes from roughly one cubic kilometre to something near eighty. All of that swallowed water does a job for the whole basin: when the Mekong peaks, the lake takes the surplus and spares Phnom Penh downstream, and when the dry months come the lake gives it back, keeping saltwater from pushing too far up the Mekong Delta.

The flood is not a disaster here. It is the engine. As the water climbs, it pushes into forests that spend half the year standing in air and half standing in water, and fish follow it in — spawning in the drowned undergrowth, sheltering there as fry, fattening on what the flood dissolves. At least 149 fish species live in the lake, eleven of them globally endangered. Among them is the Mekong giant catfish, one of the largest freshwater fish on Earth; a fisherman in Thailand landed one weighing 293 kilograms (646 pounds) in May 2005, and the species has been declining since the mid-1970s. Grey-headed fish eagles work the margins. Siamese crocodiles, endangered nearly everywhere, still hold on. UNESCO named the whole system a Biosphere Reserve in 1997, and the citation was not sentimental — few freshwater ecosystems anywhere are this productive.

The lake and its floodplain feed and employ well over a million people, and Cambodians get the great majority of their animal protein from freshwater fish, most of it from here. What can't be sold fresh becomes prahok, the pungent fermented fish paste that anchors Khmer cooking. Whole villages float on the water and move with the shoreline as it advances and retreats. Many of the families who fish these waters are ethnic Vietnamese whose ancestors have lived on the Tonlé Sap for generations, and most of them hold no citizenship papers — by one widely cited estimate, around ninety percent are stateless. Without documents they cannot own land, which is part of why they live on boats at all. Their families were hunted during the Khmer Republic years and driven out under the Khmer Rouge; after 1979 they came back to the water and went on fishing.

None of Angkor makes sense without this lake. The largest pre-industrial settlement complex known anywhere in the world grew on its northern rim, and it grew there because the flood delivered protein on a schedule reliable enough to underwrite kings, temples and armies. That relationship survives in the calendar. At the end of the rice season, villagers repair the long racing canoes kept in temples along the river, or build new ones when the old hulls are past saving. When the water turns in November, the boats race for two days, and the festival honours the naga — the serpent of Khmer cosmology, credited with spitting the lake back out toward the sea and driving the fish down the Tonlé Sap River into the Mekong. It is a myth that describes the hydrology almost exactly.

The pulse is weakening. Hydropower dams on the Mekong's upper reaches in southern China and on its tributaries in Laos hold back water in the wet season and release it in the dry, flattening the very cycle the lake depends on. In July 2020, with El Niño compounding upstream impoundment, the water level hit a record low for that point in the year — the lowest in six decades. Meanwhile the flooded forest itself is being cleared for farmland and firewood, with net forest loss recorded inside the protected areas between 1992 and 2019. Some of it is being put back: Conservation International's Cambodia programme reported in 2024 that it had restored about 600 hectares of flooded forest and planted more than 270,000 seedlings since 2010. Against a basin-scale problem, that is a small number. It is not nothing.