The annual Cambodian Water Festival takes place in October or November, at the time of the full moon, and is the largest and most exuberant festival in the country.
Millions of people from all over the country flock to the banks of the Tonle Sap river and Mekong River in the capital Phnom Penh to watch traditional boats racing.
The festival marks the reversal of flow between the Tonle Sap and Mekong River.
It is at this time when the river flow reverts to its normal down-stream direction. In a remarkable phenomenon, the Tonle Sap River earlier reverses its course as the rainy season progresses, with the river flowing "upstream" to Tonle Sap Lake.
The boat racing dates back to ancient times marking the strength of the powerful Khmer marine forces during the Khmer empire.
The celebration is also seen as a way of giving thanks to the rivers for providing the country with fertile land and abundant fish.