The Tonle Sap is the largest freshwater lake in Southeast Asia and does something no other lake does at this scale. In the dry season it drains north to south into the Mekong and covers about 2500 square kilometres at a depth of a metre. When the monsoon swells the Mekong, the flow reverses, water pushes back up the Tonle Sap river, and the lake expands to some 16,000 square kilometres and nine metres deep.
Why it matters
The flooded forest around the margins becomes a nursery, and the lake supplies well over half the protein eaten in Cambodia and around 400,000 tonnes of fish a year. The Angkorian empire was built on this cycle, and the rice surplus it allowed is what paid for the temples. It is a UNESCO biosphere reserve.
The villages
Kampong Phluk, an hour from Siem Reap, has houses on stilts up to ten metres high; in the dry season you walk under them and in the wet you arrive by boat at first-floor level. Kampong Khleang is larger, further away and much less visited. Chong Kneas is closest to Siem Reap and the most heavily commercialised, with a long record of pressure selling and staged encounters — it is the one to avoid.
Prek Toal
At the north-western corner, a core area of the biosphere reserve and the most important waterbird colony in Southeast Asia: spot-billed pelican, greater and lesser adjutant, milky stork, painted stork. Visits are by boat with a ranger, best from December to February when water levels concentrate the birds, and require an early start.
Ethics
Many residents are ethnic Vietnamese without Cambodian citizenship, which limits their access to land, schooling and work. Tours have a history of taking visitors to orphanages and schools where donations are solicited; do not visit orphanages, and buy rice or supplies through a school rather than handing money or sweets to children. Choose community-based operators and ask where the boat fee goes.
The threat
Upstream dams on the Mekong and its tributaries, together with sand extraction and climate variability, have weakened the flood pulse. Several recent years have seen the reversal arrive late and weakly, with measurable falls in the fish catch. It is one of the clearest examples anywhere of an upstream decision changing a downstream way of life.
Practicalities
Half-day trips from Siem Reap cost 20 to 35 US dollars including transport and boat, plus a government entry fee. Water levels are highest from September to November, when the villages look most dramatic; from March to May the lake retreats a long way and boats can only reach the edge.