A Unique Hydrological System

The hydrological regime of the the Tonle Sap Great Lake is extremely unique.  In Phnom Penh the Tonle Sap River meets the Mekong River.  From October to June the Tonle Sap River brings water from the lake into the Mekong River.  However in June, with the onset of the rainy season, the Mekong River delta becomes flooded, causing the Mekong to rise.  The rise of the Mekong is significant enough to cause the Tonle Sap River to reverse and flow back into the Tonle Sap Lake.

This phenomenon causes the depth of the lake to increase from 1 meter to 10 meters, and expands the total surface covered by the lake from 2,500 km² to 16,000 km².  An extensive forest area is flooded becoming wetlands.  This area provides habitat to over 200 aquatic plant species, as well as a high biodiversity of fish, reptiles, birds and mammals. This biological richness is reflected in the enormous productivity of the Tonle Sap as a fishing area: the lake supports one of the most productive fisheries in the world, and provides 40-70% of the protein intake of Cambodia's population.  In October with the beginning of the dry season the river reverses its flow again and the lake starts draining