The Tonlé Sap (Khmer: ទន្លេសាប IPA: [tunleː saːp], "Large Fresh Water River", but more commonly translated as "Great Lake") is a combined lake and river system of major importance to Cambodia.
The Tonlé Sap is the largest freshwater lake in Southeast Asia and is an ecological hot spot that was designated as a UNESCO biosphere in 1997.
The Tonlé 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. 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.