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The Tonle Sap or in Cambodian, "Large Fresh Water River", but more commonly translated as "Great Lake" is a combined lake and river system of major 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 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. From November to May, Cambodia's dry season, the Tonle Sap drains into the Mekong River at Phnom Penh. However, when the year's heavy rains begin in June, the Tonle Sap backs up to form an enormous lake.