State of water : Cambodia
Home About WEPA Activities / Documents Partner Countries Domestic Wastewater Treatment Climate Change and the Water Environment
POLICIES > State of water : Cambodia
Cases Law enforcement Background Legislative framework Organizational arrangement State of water

State of water environmental issues
Cambodia + Table of contents
+ Overview
+ River Basins
+ Sea Areas
+ Groundwater
+ Drinking Water Sources
+ Enclosed water bodies

1.4. Hydrology

Cambodia's unique hydrological regime is determined by the Mekong River and Tonle Sap Great Lake systems. Sections of the Mekong River, namely Khone Falls, are quite complex in its hydraulic performance. This section includes extensive floodplains, a complex delta and the well-known Tonle Sap Great Lake.

At the end of the dry season (November to April), the water flow in the Mekong River is at its lowest and is contained completely within the channel. At that time, the Tonle Sap Great Lake also reaches its lowest level and is only a meter or so deep. As the monsoon rains begin, river levels start to rise and flood the wetland close to the Mekong River. Local inflow also commences. As the level of the Mekong River at Phnom Penh Municipality continues to rise, the flow in the Tonle Sap River reverses and water flows from the Mekong River to the Tonle Sap River joining with local runoff to fill the Tonle Sap Great Lake to a depth of around 10 meters.

The flow into the Tonle Sap Great Lake continues for several months until the dry season commences. The flow in the Mekong River gradually decreases step by step until the Mekong's water level at Phnom Penh Municipality is lower than the water level in the Tonle Sap Great Lake. The flow in the Tonle Sap River then reverses, and starts flowing toward Phnom Penh Municipality where it supplements the flow in the Mekong River. The flow in the Tonle Sap River continues until the end of the dry season. The depth of the Tonle Sap Great Lake again lowers to around one meter.

As the Tonle Sap Great Lake fills, both from the Mekong River and local inflow, it increases in area from about 3,000 km² at the end of the dry season to about 16,000 km² at the end of the wet season. The table below indicates the source of water that can be flooded to the Tonle Sap Great Lake.

Table2: Water sources caused flooding the Tonle Sap Great Lake

Sources of flooded water Percent %
Mekong River 62
Local Inflow 38
Total 100
Source: Mekong River Commission (MRC, 1998)

This unique hydrological system is central to the national economy that supports fisheries, agriculture and water transportation. The inundation of flooded forests surrounding the Tonle Sap Great Lake during the wet season brings a large amount of external organic matter into the aquatic system, resulting in plankton blooms and surges in fish productivity. Substantial fish migrations from the Tonle Sap Great Lake during the dry season help restock fisheries in the Mekong River and many tributary rivers. Whereas soil fertility is generally poor in most of the country, the annual flooding of agricultural land leaves soil close to the Mekong River's basin fertile.


  • Mekong River Commision. 1998.
  • National Institute of Statistics. Ministry of Planning.Cambodia Statistical Year Book 2000.
  • FAO.1994

next »

Table of contents > 1. Overview
1.0. Background Information
1.1. Physical geography
1.2. Topography and administrative boundaries
1.3. Weather
1.4. Hydrology