The Tonle Sap Lake, the largest freshwater lake in Southeast Asia, is an incredible natural phe­nomenon that provides fish proteins and irrigation waters for close to half the population of Cambodia.

The lake is linked to the Mekong at Phnom Penh by a 100km-long channel, also known as the Tonle Sap (tonle meaning 'river'). From mid-May to early October (the wet season) the level of the Mekong rises rapidly, backing up the Tonle Sap river and causing it to flow northwest into the Tonle Sap lake. During this period, the lake swells from 2500 sq km to 13,000 sq km or more, its maximum depth increasing from about 2.2m to more than 10m. Around the start of October, as the water level of the Mekong begins to fall, the Tonle Sap River reverses its flow, draining the waters of the lake back into the Mekong.

This extraordinary process makes the Tonle Sap Lake one of the world's richest sources of freshwater fish, as flooded forest makes for a fertile spawning ground. Experts believe fish migra­tions from the Tonle Sap Lake help to restock fisheries as far north as China. The fishing industry supports about one million people in Cambodia and an individual's catch on the great lake can average 100kg to 200kg per day in the dry season.

This unique ecosystem has helped to earn the Tonle Sap lake protected biosphere status, but this may not be enough to protect it from the twin threats of upstream dams and exces­sive deforestation. The dams hold uncertain consequences for flow patterns of the Mekong and migratory patterns of fish. Illegal logging loosens topsoil in upland Cambodia and this silt is carried down the country's rivers into the lake. The shallowest areas may in time begin to silt up, bringing disastrous consequences not only for Cambodia, but also for neighbouring Vietnam. Hopefully, action will be taken to do all that is possible to protect this unique natural wonder from further harm, but with the Cambodian population growing by 300,000 a year, the task is going to be a far from easy.

For more information about the Tonle Sap lake and its unique ecosystem, consider visiting the exhibition about the TonIe Sap, and the Gecko Environment Centre, both in Siem Reap.