Why the Tonle Sap river reverses direction every year
Each year, the Tonle Sap river changes direction, flowing into the lake in the rainy season and out again in the dry season. Here's how it happens and why Cambodians celebrate it.
In this guide
Few rivers in the world reverse direction. The Tonle Sap does it every year, and the event is so important to Cambodia that it is celebrated with one of the country's biggest festivals.
The geography
The Tonle Sap river links the Tonle Sap lake with the Mekong at Phnom Penh. For much of the year, it flows from the lake down to the Mekong, as you'd expect.
What happens in the rainy season
From around May or June, monsoon rains and snowmelt from the Tibetan plateau swell the Mekong. At Phnom Penh, the Mekong rises so high that its water pushes back up the Tonle Sap river. For several months, the river flows north-west into the lake, filling it and flooding the surrounding plains and forests.
The reversal
When the rains ease, usually around October or November, the Mekong's level drops. The water stored in the lake then begins to drain back down the Tonle Sap river towards Phnom Penh. The river resumes its normal direction for the dry season.
Why it matters
The annual flooding:
- spreads fertile sediment over farmland
- creates huge nurseries for fish in the flooded forest
- feeds the fisheries that provide much of Cambodia's protein
- replenishes water supplies for the dry season
Bon Om Touk: the Water Festival
The Water Festival, held at the full moon in the Khmer month of Kadeuk (usually November), coincides with the time when the river returns to its normal flow. In Phnom Penh, long boat races take place on the river in front of the Royal Palace, and the city fills with visitors from the provinces. Celebrations include illuminated boats, fireworks and offerings to the moon.
Changes to the cycle
Scientists monitor the timing and strength of the reversal closely. Dams upstream on the Mekong, changing rainfall and land use all affect how much water reaches the lake. Weaker floods could reduce fish catches and harm the ecosystem, which is why the Tonle Sap is a focus of research and conservation.
Seeing it for yourself
- Watch the river from the Phnom Penh riverside, where the Tonle Sap and Mekong meet.
- Take a boat to a stilt village in the late rainy season, when the lake is at its fullest.
- Visit during the Water Festival, but plan ahead, as Phnom Penh becomes very crowded.