Le Tonlé Sap, un lac unique

The Tonle Sap Lake is one of the largest freshwater lakes in Asia, swelling to an expansive 12,000 km2 during the wet season. During the dry half of the year the Lake shrinks to as small as 2500 km2, draining into the Tonle Sap River, which meanders southeast, eventually merging with the Mekong River at the ‘chaktomuk’ confluence of rivers opposite Phnom Penh. But during the wet season a unique hydrologic phenomenon causes the river to reverse direction, filling the lake instead of draining it. The engine of this phenomenon is the Mekong River, which becomes bloated with snow melt and runoff from the monsoon rains in the wet season. The swollen Mekong backs up into the Tonle Sap River at the point where the rivers meet at the ‘chaktomuk’ confluence, forcing the waters of the Tonle Sap River back upriver into the lake. The inflow expands the surface area of lake more than five-fold, inundating the surrounding forested floodplain. The seasonally inundated forests and grasslands are unique and very important for numbers of birds, but they are threatened by the agriculture.