The Tonlé Sap (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 hotspot that was designated as a UNESCO biosphere in 1997. 

         For most of the year the lake is fairly small, around one meter deep and with an area of 2,700 square km. During the monsoon season, however, the Tonle Sap river which connects the lake with the Mekong river reverses its flow. Water is pushed up from the Mekong into the lake, increasing its area to 16,000 square km and its depth to up to nine meters, flooding nearby fields and forests. The floodplain provides a perfect breeding ground for fish.

         The pulsing system with the large floodplain, rich biodiversity, and high annual sediment and nutrient fluxes from Mekong makes the Tonle Sap one of the most productive inland fisheries in the world, supporting over 3 million people and providing over 75% of Cambodia's annual inland fish catch and 60% of the Cambodians' protein intake. At the end of the rainy season, the flow reverses and the fish are carried downriver.

         National and local observers often state that the Tonle Sap Lake is rapidly filling with sediment. However, recent long-term sedimentation studies show that net sedimentation within the lake proper has been in the range of 0.1-0.16 mm/year since ca. 5500 years before present (BP). Thus, there is no threat of the lake filling up with sediment. On the contrary, sediment is not a threat to the lake but an important part of its ecosystem, providing nutrients that drive the floodplain productivity.

         The reversal of the Tonle Sap river's flow also acts as a safety valve to prevent flooding further downstream. During the dry season (December to April) the Tonle Sap Lake provides around 50% of the flow to the Mekong Delta in Vietnam. The lake occupies a depression created due to the geological stress induced by the collision of the Indian subcontinent with Asia.

~ from Wikipedia

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