Tonlé Sap refers to a seasonally inundated freshwater lake, the Tonlé Sap Lake and an attached river, the 120 km long Tonlé Sap River, that connects the lake to the Mekong River.
They form the central part of a complex hydrological system, in the 12,876 km2 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 a unique flow reversal of the Tonle Sap River.
The Tonlé Sap Lake occupies a geological depression 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, a volume of 1 km3 and a length of 160 km at the end of the dry season in late-April to an area of up to 16,000 km2, a volume of 80 km3 and a length of 250 km as the Mekong maximum and the peak of the southwest monsoon’s precipitation 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 supply.
It proved capable of largely maintaining the Angkorean civilization, the largest pre-industrial settlement complex in world history.
Directly and indirectly it affects the livelihood of large numbers of a predominantly rural population.
Due to 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.