Tonle Sap (; Khmer: ទន្លេសាប, Tônlé Sab [tɔnleː saːp]; lit. 'Fresh River' or commonly translated as 'Great Lake') is a lake in central Cambodia. Belonging to the Mekong river system, Tonle Sap is the largest freshwater lake in Southeast Asia and one of the most diverse and productive ecosystems in the world. It was designated as a Biosphere Reserve by UNESCO in 1997 due to its high biodiversity. The lake has a large seasonal variation in water depth, with the post-monsoon level sometimes being as much as 10 metres (33 ft) higher than during the monsoons. During the 21st century, the lake and its surrounding ecosystems have come under increasing pressure from deforestation, intensive agriculture and fishery, infrastructure development and climate change.
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. The sediments of Tonle Sap also show signs of marine influence. The current river morphology of the Mekong Delta was developed over the past 6,000 years, while the remaining waters in the northwest corner of the lower Mekong plain formed the Tonle Sap. The Cambodian floodplain is mostly less than 100 meters in elevation, but a few higher outcroppings are scattered throughout the plain and much of northern Cambodia is characterized by rolling and dissected plains between 100 and 200 meters elevation. The plain is the result of erosion and sedimentation. The sediments vary in depth from at least 500 meters near the mouth of the Mekong to only about 30 meters at Phnom Penh, with bedrock outcroppings in isolated hills above the plain in several places. Hartung & Koeberl (1994) suggest Tonle Sap could be an impact crater left from an impact that had a relatively low angle, and the possible source of the Australasian strewnfield, though this hypothesis has been widely discarded.
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). The maximum area is 16,000 square kilometres (6,200 sq mi), and the volume is about 80 cubic kilometres (19 mi3). About 34% of the water in the Tonle Sap comes from the rivers that enter the lake, about 53.5% from the Mekong, and 12.5% from precipitation. May to October is the rainy season in the lower Mekong plain, and November to March is the dry season. The annual rainfall is 1,000 to 4,000 millimetres (39 to 157 in). Almost all the precipitation is in the rainy season. At the end of the dry season, the Tonle Sap Lake has a typical depth of 1 metre (3.3 ft). As the monsoon rain begins, the water level of the river begins to rise, eventually reversing the flow of the river. The water level of the Tonleé Sap increases by about 10 metres (33 ft), the flow of the Mekong gradually decreases at the end of the rainy season, and the flow of the Tonle Sap then reverses and begins to replenish the flow of the Mekong. The reversal of the Tonle Sap River's flow also acts as a safety valve to prevent flooding further downstream. During the dry season (December to April) Tonle Sap provides around 50% of the flow to the Mekong Delta in Vietnam. The hydrodynamic complexity of the Tonle Sap Lake, both in time and space, makes it impossible to measure specific flow, and water level rather than velocity and volume determines the movement of water as it shapes the landscape. 72% of the modern sediments deposited in the Tonle Sap come from the Mekong, while only 28% come from the catchments upstream of the lake. Sediment-bound phosphorus acts as the basis of the food chain through phytoplankton, and internal nutrient cycling plays a crucial role in the productivity of the floodplain and therefore the long-term sustainability of the lake's entire ecosystem.
Volume fluctuation and flow reversal
Inflow starts in May or June with maximum rates of flow of around 10,000 m3/s (350,000 cu ft/s) by late-August and ends in October or November, amplified by precipitation of the annual monsoon. In November the lake reaches its maximum size. The annual monsoon coincides to cease around this time of the year. As the Mekong River is at its minimum flow around this time of the year and its water level falls deeper than the Tonle Sap Lake, the Tonle Sap River and surrounding wetlands, waters of the lake's basin drains via the Tonle Sap River into the Mekong. As a result, the 115-kilometre-long (71 mi) Tonle Sap River flows six months a year from southeast (Mekong) to northwest (lake) and six months a year in the opposite direction. The mean annual reverse flow volume in the Tonlé Sap is 30 km3 (7.2 mi3), or about half of the maximum lake volume. A further 10 percent is estimated to enter the system by overland flow from the Mekong. The Mekong branches off into several arms near Phnom Penh and reaches Vietnamese territory south of Koh Thom and Loek Daek Districts of Kandal Province.
… excerpt ends here. Continue reading the full article.
