The Tonlé Sap (Khmer: ទន្លេសាប;), i.e., large body of water (Cambodian meaning Large Fresh Water River, but more commonly translated as Great Lake) is a combined lake and river system of huge importance to Cambodia. It is the largest freshwater lake in South East Asia and is an ecological hot spot that was designated as a UNESCO biosphere in 1997.
The adam is unusual for two reasons: 1) its flow changes direction twice a year, and 2) the portion that forms the lake expands and shrinks dramatically with the seasons. From November to May, Cambodia's dry season, the Tonlé Sap drains into the Mekong River at Phnom Penh. However, when the year's heavy rains begin in June, the Tonlé Sap backs up to form an enormous lake.