The Tonle Sap is the largest freshwater lake in Southeast Asia and also is one of the largest freshwater lakes in the world. This is an ecological hot spot that was designated as a UNESCO biosphere in 1997.
This great lake Tonle Sap is unusual for two reasons: its flow changes direction twice a year, and the portion that forms the lake expands and shrinks dramatically with the seasons. In the dry season in Cambodia (from November to May), the Tonle Sap drains into the Mekong River at Phnom Penh, but when the heavy rains season begin in June, the Tonle Sap backs up to form an enormous lake.
Source: Wikipedia