NASA satellite image of the Tonle Sap (the Great Lake)

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. 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 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. This provides a perfect breeding ground for fish. At the end of the rainy season, the flow reverses, and the fish are carried downriver. Fish from the lake and river make up 60% of the Cambodians' protein intake.

Further, as the water recedes it leaves rich deposits of sediment in the surrounding area which becomes prime agricultural land for the rest of the year.

The reversal of the Tonle Sap river's flow also acts as a safety valve to prevent flooding further downstream.

The lake occupies a depression created due to the geological stress induced by the collision of the Indian subcontinent with Asia.

Reference

• Milton Osborne, The Mekong, Turbulent Past, Uncertain Future (Atlantic Monthly Press, 2000) ISBN 0871138069