page_title: Tonlé Sap
section_title: Lower Mekong basin
hierarchical_section_title: Tonlé Sap / Lower Mekong basin
caption_attribution_description: English: Tonle Sap inhabitants
context_page_description: Tonlé Sap is 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 continuously 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 has been induced by the collision of the Indian Plate with the Eurasian Plate.
context_section_description: The lower Mekong basin (LMB) encompasses the lower half of the Mekong River's length of 2,600 km (1,600 mi) and the surrounding drainage basin of around 607,000 km² (234,000 sq mi). It is divided into five distinct physiographic units: the northern highlands; the Annamite chain; the southern uplands; the Korat Plateau; and the Mekong plain. These regions each have distinct geology, climate, vegetation and human-use patterns.
  The lower Mekong basin rests on an ancient block of continental crust that has remained relatively stable since the Jurassic Period, a period of over 150 million years. Large areas were covered by an inland sea during the upper Mesozoic Era, during which thick red-bed sandstones and evaporites were deposited.
  The Sekong, Sesan, and Srepok Rivers—collectively called the 3S Basin—are the dominant tributaries. All three enter the Mekong on the eastern bank. At the southern end of the basin, the mainstream breaks up into a complex network of branching and reconnecting channels. One of these branches forms the Tonle Sap River. The western and central parts of the southern basin make up the great lake.
  The lower Mekong basin experiences two monsoon periods: rain from May to October and a dry season from November to March. The annual rainfall is between 1,000 and 3,000 mm, locally up to 4,000 mm, falling almost entirely during the rainy season. Water levels reach their highest in September and October.
caption: Tonle Sap inhabitants
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