NASA satellite image
|Primary outflows||Tonle Sap River|
|Surface area||2,700 km² (normal)
16,000 km² (monsoon)
The Tonlé Sap is unusual for two reasons: its flow changes direction twice a year, and 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 at Phnom Penh. However, when the year's heavy rains begin in June, the Tonlé Sap backs up to form an enormous lake.
The Tonlé Sap Lake is linked to the sea via the Tonlé Sap River, which converges with the massive Mekong River in Phnom Penh. Tonlé comes from the Greek word “thalassa,” which means sea. The Tonlé Sap is comparable to sea, except that it is closed off and contains freshwater. Water has always been an important resource for Cambodia, as it is the origin of its creation. According to legend, the Khmer people were colonized in the first centuries by peaceful neighbors from India, and the combination of the two cultures eventually formed the kingdom of Cambodia.Researches have found drawings of fish etched on temple walls in such elaborate details that they could be classified as well as etchings of men with nets. To further highlight the importance of water, Cambodia’s capital city, Phnom Penh, was built at the convergence of the Tonlé Sap River and the Mekong River. Still today, the harbor resembles the descriptions recorded by first explorers, and a boat adventure from Phnom Penh is reported to be the best way to explore the river and experience Cambodian culture. 
For most of the year the lake is fairly small, around one metre deep and with an area of 2,700 square km. When water is pushed up from the Mekong into the lake, it increases its area to approximately 16,000 square kilometers, with a depth of nine meters. This expansion floods the nearby fields and forests, providing a great breeding ground for fish. 
Aside from the seasonal expansion and shrinking of the river, the Tonlé Sap is also unusual because the direction of flow changes twice a year. At the end of October, the Mekong River collects melted water from the Himalayas as well as heavy monsoon rains from the five countries it flows through, swelling massively. When it converges with the Tonlé Sap in the Cambodian capital, the volume actually forces the Tonlé Sap to reverse to flow upstream. At the start of the dry season, the Mekong River water levels drop, forcing the Tonlé Sap River to flow in its usual seaward direction, emptying the river out. May to October is wet season in Cambodia, bringing 75% of Cambodia’s rainfall. Dry season is from October to April.
This celebration also goes by Water and Moon Festival and was established to mark the reversal of the Tonlé Sap and open the fishing season. The festival lasts three days and begins on the last day of the full moon. However, because of the variation of the monsoon seasons, the reversal of the river does not always coincide exactly with the festival. In the simplest form, the celebration is a series of canoe races, including some 375 teams, and victory brings good fortune for the coming fishing season for the entire village. In addition, these water celebrations are a tribute to one of the Buddhist teeth that Naga, whose daughter married an Indian prince to establish the kingdom of Cambodia, lost in the depths. According to legend, when he was cremated, his tooth fell into the river down to the seven-headed snakes kingdom.
In pagodas along the river, men prepare for the festival by either restoring sacred canoes that have existed for hundreds of years or building new canoes when the old ones are beyond repair. Canoes are made from one piece of a trunk of a coki tree, which is essential because the material is resistant from rotting. Additionally, each canoe is personalized with painted patterns and eyes that symbolize the guardian goddess, often the spirit of a young village girl. This is a modification from the superstitious tradition of sacrifice of nailing actually eyes to the boat, dating back before Buddhism. The morning after completion and after three sacred shouts by the crew, the canoes are pushed into the river and head for the capital at full moon. Some crews must row for hours, and others will row for several days. Being chosen as a member of the crew is one of a man’s highest honors, and members must practice to perfect team coordination. Only the best crews will get to the finals in the capital.
After two days of racing, all of the canoes come together to encourage Naga to spit out the swelling waters of the Tonlé Sap towards the sea. Firecrackers light the water, the royal palace, and the sky. This moment lets the legendary snake master of water know to return to the depths of the Tonlé Sap and leave the power to the sun gods. This also marks the end of the rainy season. 
When the Tonlé Sap floods, the surrounding areas become a prime breeding ground for fish. During this time, fishermen are scarce. Fishing during this time is actually illegal, as to prevent disruption of mating. At the end of the rainy season, when the water levels go down, fishing is allowed again. Fisherman install floating houses along one half of the river, and the other half is left open for navigation.
Most of the fishing captains are of Vietnamese origin, and they primarily supply the country’s markets. Fisherman Sakaloy explains, “My parents were fisherman. We have lived in Cambodia and have this activity for a long time. We started well before the poor part era, when the Khmer Rouge took over from 1975 to 1979. We had to flee to Vietnam, [but] afterwards we came back and have been fishing on the Tonlé Sap ever since.” However, thousands of peasants follow this lifestyle hoping for an incredible catch, making it difficult to find spaces big enough to settle with all of the village carts. 
The process of actually catching fish is simple, but the aftermath takes much more time and effort. Because the drop in the water level, the Tonlé Sap naturally carries away thousands of fish. The fishermen simply cone-shaped nets into the water from their floating houses, and then lifting the net as soon as seconds later. Using this technique, two or three tons of fish are trapped each time and more than ten thousand tons of fish can be caught in under a week. One by one, fishermen, mostly women, cut off the fish heads then bring the fsh back to the river to be cleaned and to remove the fat. Salting the fish for preservation is the final step in this process, but the fish will continue to macerate for several months in order to transform into a paste called prahok, a nourishing condiment that compliments almost any dish. These couple of days, on average three days, of fishing supplies enough prahok for the entire year.
Fishermen use all parts of the fish for their own needs and also for profit. The removed heads of the fish are dried in the sun, becoming a good fertilizer they can sell. This small amount of extra money is acquired for cases of emergency, such as family illness. By boiling the fat from the bottom of the fish basket, fishermen can also make soap for their personal use. Through bartering on the banks, they exchange fish for rice. Excess rice is also sold for profit. Despite paying employees and buying an official fishing license, fishermen still “have enough money to feed my family for a year. So I don’t need rice fields.” To further emphasize the importance of fish in the local economy, the name given to the Cambodian form of currency is Riel, which is a small silver carp that is the staple of most diets.
The implication of the fishing industry in Cambodia is the country’s strong connection to religion, specifically Buddhism. Because of the moral consequences of taking a life, Buddhists limit Cambodian fishing to the amount necessary to feed their families. Additionally, to further lessen the guilt, fishermen do not actually kill the fish; instead they wait for it to die naturally when taken out of the water. Even so, fishermen go to pagodas after the fishing season for purification.
The beginning of the dry season is also the beginning of rice season, which is the only source of wealth for peasants. A good harvest will provide enough rice for them to survive for the entire year, but if the floods are too big or too small, rice can become scarce. Because of this uncontrollable instability, many celebrations are held in honor of gods and genies that can influence nature and bring about a good harvest.
Historical research has shown that the old Angkorian civilization took advantage of the weather conditions by digging huge reservoirs during the wet season and releasing the water during the dry season using an irrigation system and the land’s natural slope. This double and sometimes even tripled the amount of rice crops per year, strengthening the developing nation. However, today, this irrigation network is no longer present, and peasants only get one rice crop a year. What has not changed is the planting of rice in fields as well as the survival value of rice. It is still the main source of income for peasants and the only currency used to bargain. They use the crops to pay for what they need, such as property rent for land to plant, and the rest is kept for the family to eat.
In the fields in villages, the harvesting and graining are done traditionally, meaning that no machinery is used. As the sun rises, villagers harvest the mature rice plants and replant new ones in their place. Harvested rice plants are cleaned, sprayed, and bundled. Women stoop in the mud with their feet for hours to replant each rice root, and as the water level decreases, a field of green is once again visible. Part of the rice cultivated is grilled, grinded, and winnowed, a ritual preparation for the upcoming celebration. Rice is also used to prepare lunch, and one meal reflective of both the fishing and rice industry is Tonlé Sap chicken. The rice and chicken is cooked in the river’s water, and prahok paste made from fish, is then mixed in to add flavor.
At the end of the rice season, villagers celebrate by marching to the pagoda in a procession. This is a chance for everyone to relax after a difficult harvesting season as well as provide an opportunity to have fun and bond with the community. All of the villagers wear their nicest clothes, musicians sing and dance, and men even take the chance to court young women. Upon arrival, believers circle the temple three times and then proceed to gift presents such as clothing, dishes, furniture, and food. These donations, also named Kathen by Buddha, provide help for bonzis, who in turn give blessings. This act of donation is essential in accumulating good karma for reincarnation and reaching Nirvana, or ultimate salvation, as well as harvests to come. 
Although the large amount of sediment in the Tonlé Sap Lake basin is a natural phenomenon, rapid rates of development and resource exploitation has caught the attention of observers who fear the basin itself is in danger of filling with sediment. These fears were first reported by local people living lakeside that noticed some areas becoming shallower. With increased sedimentation, already vague transit routes between capital and regional centers would likely be shut down altogether and may restrict the migration of fish into the lake.
Because sediment contains nutrients that fuel food webs, the Tonlé Sap is actually benefitting from the influx. Sediment-bound phosphorus serves as food for phytoplankton through higher plants, and research has shown that the metabolizing of the chemical contributes to food abundance and quality. Internal nutrient cycling, therefore, plays an essential role in productivity of a floodplain. The nutrients bound to suspended sediments are important for the Tonle Sap system, particularly to maintain its long-term sustainability.
The reversal of the Tonlé Sap river's flow also acts as a safety valve to prevent flooding further 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, there have been concerns from scientists about the building of high dams and other changed hydrological parameters in Southern China and Laos that has threatened the strength and volume of the reverse flow into Tonle Sap, which in turn decreases nesting, breeding, spawning, and feeding habitats in floodplain, which results in adverse impacts on fish productivity and overall biodiversity.
The river is home to at least 149 species of fish, 11 globally threatened species, and 6 near-threatened species. These species include the spot-billed pelican, greater Adjutant, Bengal Florican, Darter, Grey-headed Fish Eagle, and the Manchurian Reed Warbler. Specifically, the large colonies of unique birds constitute the The Preak Toal Bird sanctuary.
In addition, the Tonlé Sap also supports significant reptile populations including nearly extinct Siamese Crocodiles and the world’s largest population of freshwater snakes. Although the area around the lake has been modified for settlement and farming, about 200 species of plants have been recorded. 
One of the most legendary species living in the Tonlé Sap is the Mekong giant catfish, the largest freshwater fish in the world. The fish is 8 to 10 feet long and can weigh anywhere between 250 and 500 pounds. The largest of these catfish ever caught weighed 674 pounds. Despite its massive physical characteristics, the Mekong catfish is especially vulnerable to chemical changes, which is beneficial in alerting authorities of trouble in the river ecosystem early on. The population of these fish has been steadily declining since the Khmer Rouge era, led by Pol Pot, and in 2005, fisherman reported that on average only one giant catfish was caught per day. Currently, it is illegal for fishermen to catch and keep these fish with the exception of a few retained by fisheries for research. It also cannot be used in any form of trade in fear of the economic exploitation.
In 1997, the United Nations Educational, Scientific and Cultural Organization, more commonly known as UNESCO, deemed Tonlé Sap an ecological hotspot. As a result, in 2001, by Royal Decree issued by the government of Cambodia, the lake and its surrounding provinces became 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.
The government is responsible for fulfilling three functions:
a) a conservation function to contribute to the conservation of biological diversity, landscapes, and ecosystem, including genetic resources, plant, fishery and animal species, and to the restoration of the essential character of the environment and habitat of biodiversity;
b) a development function to foster sustainable development of ecology, environment, economy, society, and culture;
c) a logistic function to provide support for demonstration projects, environmental education and training, research and monitoring of environment related to the local, national and global issues of conservation and sustainable development.
Additionally, the Tonlé Sap Biosphere Reserve established three zones: a core zone, a buffer zone, and a transition zone. Formally, the core area of a Biosphere Reserve is defined as an area devoted to biological resources, landscapes, and ecosystems. The core zone includes practices that protect sites for conserving biodiversity, monitoring minimally disturbed ecosystems and undertaking non-destructive research and related activities. As of today, the three zones are Prek Toal, Boeng Chhmar, and Stung Sen.
Despite this government protection, illegal fishing, poaching, and cutting of the forest for farmland are all still major problems. Because people living around the lake are extremely poor and depend on the lake for their survival, it is likely that this unsustainable living will continue. During recent years, the amount of fish caught has been steadily declining, which means peasants must also work harder to provide for their families. The government is working on supporting and educating these people to break this cycle of poverty and unsustainability. Finding a balance between survival and conservation seems to be the major question for the future.
The area is home to many ethnic Vietnamese and numerous Cham communities, living in floating villages around the lake. Approximately 1.2 million people living in the greater Tonle Sap make their living by fishing on the local waters. Cambodia produces about 400,000 tonnes of freshwater fish per year, the majority of which comes from Tonle Sap. These fisheries account for 16 percent of national GDP, making the fish industry not only essential to the diet of local populations but to the Cambodian economy as a whole.
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