They form the central part of a complex hydrological system, situated in the 12,876 km2(4,971 sq mi) Cambodian floodplain covered with a mosaic of natural and agricultural habitats that the Mekong replenishes with water and sediments annually. The central plain formation is the result of millions of years of Mekong alluvial deposition and discharge. From a geological perspective, the Tonlé Sap Lake and Tonlé Sap River are a current freeze-frame representation of the slowly, but ever shifting Lower Mekong Basin. Annual fluctuation of the Mekong's water volume, supplemented by the Asian Monsoon regime causes the unique flow reversal of the Tonle Sap River.
The Tonlé Sap Lake occupies a geological depression (the lowest lying area) of the vast alluvial and lacustrine floodplain in the Lower Mekong Basin, which had been induced by the collision of the Indian Plate with the Eurasian Plate. The lake's size, length and water volume varies considerably over the course of a year from an area of around 2,500 km2 (965 sq mi), a volume of 1 km3(0.24 cu mi) and a length of 160 km (99 mi) at the end of the dry season in late April to an area of up to 16,000 km2 (6,178 sq mi), a volume of 80 km3 (19 cu mi) and a length of 250 km (160 mi) as the Mekong maximum and the peak of the South-West monsoon'sprecipitation culminate in September and early October.
As one of the world’s most varied and productive ecosystems the region has always been of central importance for Cambodia's food provision. It proved capable of largely maintaining the Angkorean civilization, the largest pre-industrial settlement complex in world history. Either directly or indirectly it affects the livelihood of large numbers of a predominantly rural population to this day. With regards to a growing and migrating population, ineffective administration and widespread indifference towards environmental issues the lake and its surrounding ecosystem is coming under increasing pressure from over-exploitation and habitat degradation, fragmentation and loss. All Mekong riparian states have either announced or already implemented plans to increasingly exploit the river's hydroelectric potential. A succession of international facilities that dam the river's mainstream is likely to be the gravest danger yet for the entire Tonle Sap eco-region.
The largest freshwater lake in Southeast Asia, that contains an exceptional large variety of interconnected eco-regions with a high degree of biodiversity is a biodiversity hotspot and was designated as a UNESCO biosphere reserve in 1997.