(Cambodian meaning "Large Fresh Water River," but more commonly translated as "Great Lake") is a combined lake and river system of huge importance to Cambodia. It is the largest freshwater lake in South East Asia and is an ecological hot spot that was designated as a UNESCO biosphere in 1997.

The Tonlé Sap is unusual for two reasons: 1) its flow changes direction twice a year, and 2) the portion that forms the lake expands and shrinks dramatically with the seasons. From November to May, Cambodia's dry season, the Tonlé Sap drains into the Mekong River atPhnom Penh. However, when the year's heavy rains begin in June, the Tonlé Sap backs up to form an enormous lake.

For most of the year the lake is fairly small, around one meter deep and with an area of 2,700 square km. During the monsoon season, however, the Tonlé Sap river, which connects the lake with the Mekong river, reverses its flow. Water is pushed up from the Mekong into the lake, increasing its area to 16,000 square km and its depth to up to nine meters, flooding nearby fields and forests. The floodplain provides a perfect breeding ground for fish.

 

Map of Tonle Sap Lake and its floodplain

The pulsing system with the large floodplain, rich biodiversity, and high annual sediment and nutrient fluxes from Mekong makes the Tonlé Sap one of the most productive inland fisheries in the world, supporting over 3 million people and providing over 75% of Cambodia's annual inland fish catch and 60% of Cambodians' protein intake. At the end of the rainy season, the flow reverses and the fish are carried downriver.

National and local observers often state that the Tonlé Sap Lake is rapidly filling with sediment. However, recent long-term sedimentation studies show that net sedimentation within the lake proper has been in the range of 0.1-0.16 mm/year since ca. 5500 years before present (BP). Thus, there is no threat of the lake filling up with sediment. On the contrary, sediment is not a threat to the lake but an important part of its ecosystem, providing nutrients that drive the floodplain productivity [citation needed].

 

Water dwelling on the lake of Tonle Sap near Siem Reap

The reversal of the Tonlé Sap river's flow also acts as a safety valve to prevent floodingfurther downstream. During the dry season (December to April) the Tonlé Sap Lake provides around 50% of the flow to the Mekong Delta in Vietnam.

The lake occupies a depression created due to the geological stress induced by the collision of the Indian subcontinent with Asia. In recent years the building of high dams in Southern China and Laos has threatened the strength and volume of the reverse flow into Tonle Sap; a phenomenon that environmentalists have been slow to recognize or raise concern about. Already fish catches are significantly down.

The Tonle Sap Lake and its surrounding provinces are part of the Tonle Sap Biosphere Reserve. There are nine provinces that are part of the Tonle Sap Biosphere Reserve, these are; Banteay Meanchey, Battambang, Kampong Chhnang, Kampong Thom, Preah Vihear, Pursat,Siem Reap, Otdar Meanchey, and Krong Pailin.

Cambodia's Great Lake, the Boeung Tonle Sap (Tonle Sap Lake,) is the most prominent feature on the map of Cambodia - a huge dumbbell-shaped body of water stretching across the northwest section of the country. In the wet season, the Tonle Sap Lake is one of the largest freshwater lakes in Asia, swelling to an expansive 12,000 km2. During the dry half of the year the Lake shrinks to as small as 2500 km2, draining into the Tonle Sap River, which meanders southeast, eventually merging with the Mekong River at the 'chaktomuk' confluence of rivers opposite Phnom Penh. But during the wet season a unique hydrologic phenomenon causes the river to reverse direction, filling the lake instead of draining it. The engine of this phenomenon is the Mekong River, which becomes bloated with snow melt and runoff from the monsoon rains in the wet season. The swollen Mekong backs up into the Tonle Sap River at the point where the rivers meet at the 'chaktomuk' confluence, forcing the waters of the Tonle Sap River back upriver into the lake. The inflow expands the surface area of lake more than five-fold, inundating the surrounding forested floodplain and supporting an extraordinarily rich and diverse eco-system. More than 100 varieties of waterbirds including several threatened and endangered species, over 200 species of fish, as well as crocodiles, turtles, macaques, otter and other wildlife inhabit the inundated mangrove forests. The Lake is also an important commercial resource, providing more than half of the fish consumed in Cambodia. In harmony with the specialized ecosystems, the human occupations at the edges of the lake is similarly distinctive - floating villages, towering stilted houses, huge fish traps, and an economy and way of life deeply intertwined with the lake, the fish, the wildlife and the cycles of rising and falling waters. 

The lake sits only about 15 km south of Siem Reap town. If you take the ferry between Phnom Penh and Siem Reap you will cross the lake and dock at the village of Chong Khneas. There are several ways to see the culture and wildlife of the lake area depending on the amount of time you have and your interest.