The Boat Races in Siem Reap during Bonn Om Toeuk. celebration.

The Water Festival also marks a unique natural phenomenon - the Tonle Sap river reverses the flow of its current. It is probably the only waterway in the world which flows in opposite directions at different times of the year.

The Tonle Sap lake or Great Lake is a vast expanse of water, once an arm of the sea, which forms the most significant topographical feature in country. The lake is fed by the Mekong river and the Tonle Sap river.

From November to May, the Tonle Sap river runs into the Mekong just like any other tributary.

But with the arrival of the monsoon rains, there is such build-up of water in the main stream that excess pours into the Tonle Sap river, forcing it to change direction and flow back into the Tonle Sap lake.