The Tonl? Sap (Large Fresh Water River or Great Lake) is a combined lake and river system of major importance to Cambodia. The Tonl? Sap 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 Tonl? 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 Tonl? Sap drains into the Mekong River at Phnom Penh. However, when the year's heavy rains begin in June, the Tonl? Sap backs up to form an enormous lake. The Tonl? Sap is home to many ethnic Vietnamese and Cham communities, living in floating villages around the lake.