Tonlé Sap – the lake whose river reverses twice a year
Tonlé Sap
The Mekong now stands higher than the lake, so the Tonlé Sap river runs backwards and pumps water in. Fish move upstream into the lake to spawn. This is when the lake expands fastest.
Mekong basin rainfall: 39 mm over the past 72 hours, 30 mm forecast for the next 72.
This is a seasonal estimate combined with basin rainfall, not a measured level. Official water level and discharge come from the Mekong River Commission (MRC). The timing of the reversal varies year to year and has shifted noticeably over the past two decades.
Why the lake reversesThe annual cycle
| Month | State | Area | Depth |
|---|---|---|---|
| January | Flow reversed again, lake draining | 5,813 km² | 3.3 m |
| February | Flow reversed again, lake draining | 4,844 km² | 2.6 m |
| March | Flow reversed again, lake draining | 3,969 km² | 2 m |
| April | Flow reversed again, lake draining | 3,000 km² | 1.3 m |
| May | Dry season, lake contracting | 3,083 km² | 1.4 m |
| June | Flow reversed, lake filling | 4,375 km² | 2.3 m |
| July | Flow reversed, lake filling | 7,099 km² | 4.1 m |
| August | Flow reversed, lake filling | 9,915 km² | 6 m |
| September | Flow reversed, lake filling | 12,730 km² | 8 m |
| October | Lake at maximum extent | 14,880 km² | 9.4 m |
| November | Flow reversed again, lake draining | 13,059 km² | 8.2 m |
| December | Flow reversed again, lake draining | 9,112 km² | 5.5 m |
Why the river reverses
The Tonlé Sap river links the lake to the Mekong at Phnom Penh. Its direction is set by the difference in water level, not by terrain. In the dry season the lake stands higher and drains out. In the wet season, snowmelt from the Tibetan plateau plus the monsoon raise the Mekong until it exceeds the lake, and the flow reverses inward. The lake works as a natural relief valve for the lower Mekong, absorbing the flood peak and releasing it slowly.
Why this decides what the country eats
As the lake rises it submerges roughly a million hectares of flooded forest around its edge. That forest is where fish spawn and fry grow. Cambodia's freshwater catch depends almost entirely on whether the lake rises high enough for long enough. Fish is the main protein source for most of the population, so a weak flood year is a protein shortage, not only an income shortage.
What is changing
The timing and amplitude of the flood have shifted noticeably over two decades. The reasons most often cited are the chain of upstream Mekong dams together with a more erratic monsoon. That is why this page estimates from actual basin rainfall rather than a fixed date — a fixed calendar would drift further wrong each year.
If you are visiting
September to November is when the lake is widest: the floating villages of Kampong Phluk and Chong Kneas are easy to reach by boat and the Prek Toal flooded forest is fully submerged. March to May the lake is low, many boat channels are unusable and the floating villages sit on mud. Bon Om Touk, around early November, marks the moment the flow turns back out.