Tonlé Sap (also known as Tonlé Sab) is a lake and river system in Cambodia that has the unusual distinction of reversing the direction of its flow twice a year.
During the dry season in Cambodia, the river flows from the lake to join the Mekong River. However, during monsoon season, from approximately May to October, the Mekong swells from rainwater and pushes water back up into the lake. The lake can increase in size from around one meter (3 feet) deep during the dry season, to nine meters (30 feet) deep during the monsoon season.
Tonlé Sap lake is an important source of freshwater fish, yielding about 300,000 tons each year.
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