The Combined Lake and River System of the Tonlé Sap, Cambodia12.9N 104.0E
The Tonlé Sap, southwest of the image center, is a combined lake and river system of huge importance to Cambodia. It is the largest freshwater lake in Souteast Asia (Cambodia, eastern Thailand and southern Laos are all visible here) and is an ecological hot spot that was designated as a UNESCO biosphere in 1997.
The Tonlé Sap is unusual for two reasons: 1) its flow changes direction twice a year, and 2) the portion that forms the lake expands and shrinks dramatically with the seasons. From November to May, Cambodia’s dry season, the Tonlé Sap drains into the Mekong River at Phnom Penh. However, when the year’s heavy rains begin in June, the Tonlé Sap backs up to form an enormous lake.
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 Tonlé 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 pulsing system with the large floodplain, rich biodiversity, and high annual sediment and nutrient fluxes from Mekong makes the Tonlé 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 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 Tonlé Sap Lake is rapidly filling with sediment, as can be observed from its tan color here. 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, some say, sediment is not a threat to the lake but an important part of its ecosystem, providing nutrients that drive the floodplain productivity.