Tonle Sap, the ‘Great Lake’ of Cambodia, is a combined lake & river system that’s of huge importance to Cambodia. It is the largest freshwater lake in SE Asia. The immense flood plain is located between Siem Reap & Phnom Penh (see map). The Tonlé (ton-lay) Sap is unusual for two reasons - its flow changes direction twice a year, & the portion that forms the lake expands & shrinks dramatically with the seasons. This phenomena starts with the Mekong river, which begins its journey in the Himalayas, then flows through China, Laos & Cambodia before discharging into the sea at its delta in South Vietnam. Since the Mekong Delta cannot absorb all of the river water, the river reverses its normal seaward flow in May & begins to flow back up the Tonle Sap River. The 160km long lake then fills with water & expands to 250kms long by up to 100kms wide over a period of 3 months. The lake rises & progressively floods the surrounding forests & rice fields. So from November to May, Cambodia's dry season (when we were there), the Tonlé Sap drains into the Mekong River at Phnom Penh, & when the monsoon season begins in June, it backs up to form an enormous lake.

This annual back-and-forth flow creates a rich breeding ground for fish, which in turn ensures a rich biodiversity for the Tonle Sap region, which is why Tonle Sap was designated as a Unesco biosphere in 1997. Tonlé Sap is one of the most productive inland fisheries in the world, supporting over 3 million people, & providing over 75% of Cambodia's annual inland fish catch, & 60% of Cambodians' protein intake. At the end of the rainy season, the flow reverses & the fish are carried downriver. 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. In recent years the building of high dams in Southern China & Laos has threatened the strength & volume of the reverse flow into Tonle Sap, something that environmentalists have been slow to recognize or raise concern about. Already fish catches are significantly down.

More info here: http://en.wikipedia.org/wiki/Tonle_Sap

Interesting photos here: http://www.peaceofangkorweb.com/TonleSap.htm

Photos of other lake villages you might prefer to visit: http://www.peaceofangkorweb.com/LakeVillages.htm

A great photo gallery, as usual: http://www.asiaexplorers.com/cambodia/tonlesap.htm