 The Royal Water Festival celebrates a rare natural phenomenon known as the reversal of the Tonle Sap River.In November, at the end of the annual monsoon, the Tonle Sap lake reaches its maximum size.As the Mekong River is at its minimum flow around this time of the year and its water level falls deeper than the Tonle Sap Lake, the Tonle Sap River and surrounding wetlands drain into the Mekong.