Tonle Sap and the lake that changes its shape

Tonle Sap is Cambodia’s Great Lake, lying south of Siem Reap in the central plain. Its defining feature is a shoreline that changes dramatically as the Tonle Sap River reverses direction and the lake expands into its floodplain.

In the wet season, water from the Mekong moves north-west through the Tonle Sap River. In the dry season, the flow runs back towards the Mekong. The same lake therefore appears as a compact freshwater basin at one time of year and a broad inland water landscape at another.

Tonle Sap: a lake with a seasonal shape

Tonle Sap is the largest freshwater lake in Southeast Asia. During the dry season, its area contracts to roughly 2,700 km², while the wet-season lake reaches about 10,360 km². Its depth rises from less than 3 m in shallow areas to between 9 and 14 m during the monsoon.

The change begins through the Tonle Sap River, which connects the lake with the Mekong near Phnom Penh. When the Mekong rises, the river’s flow turns towards the lake. When floodwaters subside, the direction reverses and the lake drains south-eastward. seasonal lake expansion is part of this annual hydrological cycle.

Flooded forests, fish and a protected basin

The lake sits at the centre of the Tonle Sap Basin, bordered by the Dong Rek Mountains to the north and the Cardamom Mountains to the south-west. Around the open water are floodplains, freshwater swamps, seasonally flooded rice fields and grasslands.

Flooded forests form a major part of the basin’s ecological character. Their trees and plants endure changing water levels, while fish use the flooded landscape as habitat. The reserve contains about 200 higher plant species and 225 bird species, including the masked finfoot and the greater adjutant. Tonle Sap Biosphere Reserve entered UNESCO’s Man and the Biosphere network in 1997.

The protected area includes the lake, surrounding floodplains and freshwater swamps. Its boundaries also contain seasonally flooded fields and grasslands. A Ramsar wetland lies within the reserve at Boeung Chhmar and its associated river system and floodplain.

Floating villages south of Siem Reap

Communities around Tonle Sap reflect the lake’s changing water level. Houses may stand on tall timber stilts above the floodplain, while other homes rest on floating platforms connected to channels and open water.

Chong Khneas, Kampong Phluk and Kampong Khleang are among the settlements associated with the Siem Reap side of the lake. Their buildings, boats, piers and waterways belong to a landscape shaped by fishing, transport and seasonal movement across water.

Residents include Khmer, Cham and Vietnamese communities. Fishing, aquaculture, rice cultivation and trade form part of the region’s social and economic life. Salted and fermented fish are also established elements of Cambodian food culture.

Visiting Tonle Sap: what travellers from Western Europe should know

• Tonle Sap lies south of Siem Reap. Boat routes from Chong Khneas and other lakeside settlements lead through channels towards the open water.
• The landscape changes with the season. Floating homes remain on the water, while stilted houses stand above ground that can be flooded during the monsoon.
• The official language is Khmer and the currency is the Cambodian riel.
• Entry requirements depend on your nationality; UK travellers can check the FCDO travel advice at gov.uk/foreign-travel-advice, Irish travellers the Department of Foreign Affairs at ireland.ie.

Searching for Tonle Sap on social platforms

These links open searches for Tonle Sap on each platform.

Frequently asked questions about Tonle Sap

Where is Tonle Sap?

Tonle Sap lies in central Cambodia, south of Siem Reap. The lake connects with the Mekong through the Tonle Sap River.

What can visitors expect around the lake?

Visitors encounter waterways, floating homes, stilted villages, flooded forests and boats moving through channels near the Siem Reap shore.

What makes Tonle Sap distinctive?

Its river changes direction with the seasonal rise and fall of the Mekong. This process expands the lake across its floodplain and contracts it again during the dry season.