Tonle Sap

Hydrology

The Tonle Sap drainage basin is bordered to the southwest by the Cardamom Mountains, with heights of more than 1,700 metres (5,600 ft), and to the north are the Piandan Mountains with an average height of 500 metres (1,600 ft). The basin covers an area of 86,000 square kilometres (33,000 sq mi). The 120 kilometres (75 mi)-long Tonle Sap River connects the Tonle Sap Lake with the Mekong River and contributes 9% of the flow of the Mekong.

The size and water volume of the lake varies greatly throughout the year, with a minimum area of about 2,500–3,000 square kilometres (970–1,160 sq mi) and a volume of about 1 cubic kilometre (0.24 mi3) in the dry season, and the water body expands in the rainy season, increasing the depth to 9–14 metres (30–46 ft).

Ecology

The land cover of the Tonle Sap basin is 55% forest and 45% agricultural land. The lake is surrounded by freshwater mangroves known as "flood forests", accounting for 3% of the basin area, and the floodplain is surrounded by low hills and covered with evergreen or deciduous seasonal tropical forest dominated by species of Dipterocarpaceae, Leguminosae, Lythraceae, Fagaceae and in some places Pinaceae, Podocarpaceae, or bamboo. Farther from the lake, the forest gradually turns into a thicket, and finally into a meadow. In areas with higher quality soils or higher altitudes, deciduous mixed forests and semi-evergreen forests occur.

Geology

Tonle Sap Lake is located in the northwest of the lower Mekong plain, formed by the collision and collapse of the Indian Plate and the Eurasian Plate. 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 lower Mekong plain used to be a bay, and the sea level rose rapidly at the end of the last glacial period. About 4.5 metres (15 ft) high, cores from this period found near Angkor contain tidal deposits, as well as salt marshes and mangrove swamp deposits, deposited in caves about 7,900–7,300 years ago.