Tonlé Sap () is a river in

Cambodia Cambodia, officially the Kingdom of Cambodia, is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. It is bordered by Thailand to the northwest, Laos to the north, and Vietnam to the east, and has a coastline ...

that connects the

Tonlé Sap Tonlé Sap (; , ; or commonly translated as 'Great Lake') is a lake in the northwest of Cambodia. Belonging to the Mekong, Mekong River system, Tonlé Sap is the largest freshwater lake in Southeast Asia and one of the most diverse and prod ...

lake with the

Mekong The Mekong or Mekong River ( , ) is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of and a drainage area of , discharging of wat ...

river. The river is known for its unusual seasonal reversal, which plays a crucial role in supporting biodiversity, sustaining local livelihoods, enhancing agricultural productivity, and contributing to the ecological, economic, and cultural vitality of the region.

Etymology

In Khmer, Tonlé/ means "river" and Sap/ means "fresh water". Therefore, Tonlé Sap () is the name of the river while the lake is called "Boeng Tonlé Sap" (, where Boeng/បឹង means "lake").

Geography

From the lake, the Tonlé Sap flows southeast to its confluence with the Mekong river near

Phnom Penh Phnom Penh is the Capital city, capital and List of cities and towns in Cambodia, most populous city of Cambodia. It has been the national capital since 1865 and has grown to become the nation's primate city and its political, economic, industr ...

, the capital of Cambodia. During the

dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The t ...

, the Mekong and Tonlé Sap rivers merge, but in the

monsoon season The wet season (sometimes called the rainy season or monsoon season) is the time of year when most of a region's average annual rainfall occurs. Generally, the season lasts at least one month. The term ''green season'' is also sometimes used a ...

(May to October), the Mekong's floods cause the Tonlé Sap river to reverse its flow. The reversal carries water, along with fish eggs and larvae, into the Tonle Sap Lake, where they find a nutrient-rich environment that supports their feeding and growth. The confluence of the Mekong, Tonlé Sap, and Bassac rivers, known as Chaktomuk, holds significant ecological and economic importance for Cambodia. It is a crucial source of drinking water for around 1.5 million residents of Phnom Penh, as well as supplying water for various industrial and commercial activities. Chaktomuk also receives both treated and untreated wastewater. As a key transportation hub in the region, it accommodates a wide range of vessels, including large sea-going cargo ships, high-speed passenger boats, and small fishing boats, all of which can dock and unload goods, materials, people, and food at the Phnom Penh Autonomous Port. This port, located along the Tonlé Sap river approximately 3-4 km upstream from the confluence, is an international facility managed by the Cambodian Ministry of Public Works and Transport and the Ministry of Economy and Finance.

Culture

The

Bon Om Touk Bon Om Touk (, , lit. "Boat Paddling Festival"), also known as the Cambodian Water Festival, is celebrated in late October or early November, often corresponding with the lunar Mid-Autumn Festival. It marks the end of the monsoon season. The f ...

, or Water Festival, is one of Cambodia's most prominent cultural events, celebrated annually on the Tonlé Sap River. This festival marks the end of the rainy season and the reversal of the river's flow, symbolizing the start of the fishing season. The highlight of the festival is the boat races, which attract thousands of participants and spectators to Phnom Penh, particularly around the Royal Palace area. The festival not only celebrates the unique hydrological phenomenon of the Tonle Sap but also underscores the river's importance to Cambodian culture and livelihoods.

Impact of upstream Mekong dams

Concerns over the impact of upstream Mekong dams on the natural reversal of the Tonlé Sap River are growing. These dams, particularly those in China and

Laos Laos, officially the Lao People's Democratic Republic (LPDR), is the only landlocked country in Southeast Asia. It is bordered by Myanmar and China to the northwest, Vietnam to the east, Cambodia to the southeast, and Thailand to the west and ...

, disrupt critical seasonal flooding patterns, leading to delays or failures in the river's annual reversal. This disruption causes the Tonlé Sap lake to expand and contract less predictably, reducing fish stocks and agricultural productivity. Consequently, the livelihoods and food security of millions of Cambodians who depend on the lake and river are threatened. Studies indicate that this disruption diminishes floodplain fertility and alters flood dynamics, emphasizing the urgent need for sustainable water management.

References

{{Rivers of Cambodia Rivers of Cambodia Tonlé Sap Geography of Phnom Penh