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The Tonle Sap Lake (also known as the Great Lake) is situated in the lowlands of central Cambodia, extending for some 250 km from northwest to southeast and is up to 100 km wide. At low water level, the lake is about 120 km long and up to 35 km wide and covers an area of 2,500 to 3,000 square kilometres. With an average minimum depth of less than one metre the lake is very shallow. It is surrounded by a broad belt of natural freshwater swamp forest, which, in turn, gives way to a broad belt of cultivated rice paddies.
The lake is connected to the Mekong River by way of the Tonle Sap River. After the onset of the rainy season in June and July, the river current reverses and carries Mekong water to the Tonle Sap Lake. Subsequently the Mekong overflows its banks and spreads laterally with some of the overland flow also flowing to the Tonle Sap River and then to the Lake. The topography of the floodplain spreads the floodwaters over wide areas, and by the end of September or early October the area of the Tonle Sap Lake has expanded to between 11,000 and 13,000 square kilometres, up to a maximum 18,000 km2. At this time, the lake has an average depth of six to nine metres. The water level falls rapidly between January and March, and the lowest levels occur in April and May. The water of the open part of Tonle Sap Lake is always extremely turbid because winds constantly churn the shallow bottom sediments. The lake receives annually around 8-9 million tonnes of sediment from its tributaries and the Mekong. Only 20% of the sediment leaves the lake during the dry season to the Mekong. Most of the sediment settles down to its floodplains as the annual net sedimentation in the open lake is only around 0.1 mm. Thus, the Tonle Sap Lake is not filling up with sediment.
The extent of the Lake’s surface was previously monitored by ground-based surveys or aerial reconnaissance; recently the analysis of satellite imagery has provided more frequent and accurate assessments.
The Tonle Sap Lake was at the heart of the ancient Khmer Empire. The famous temples at Angkor are located around the town of Siem Reap, on the North-Eastern side of the lake. Then, as now, the lake supports one of the world's most productive freshwater fisheries, providing a large proportion of the nutritional requirement of the human population. The combined fish yield of the lake and the river is estimated to be around 100-150 kg per ha per year. The high yield is explained by three inter-related factors – numerous fish species, large areas of seasonally inundated flood plains, and increased fishing activity. The inundation zone of the Tonle Sap Lake and Tonle Sap River is a vital spawning and nursery area for a wide variety of fish species. The lowland delta fishery derives much benefit from the aquatic production in this floodwater complex.
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