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How is the Mekong River connected to Tonle Sap?
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Mekong River
The Irrawaddy and Mekong river basins and their drainage networks.
The Mekong River and the Tonle Sap are connected by the Sab River, which acts as a unique, reversible channel between the two bodies of water.12
This connection is defined by an annual hydrological cycle driven by the monsoon season:
- Rainy Season (Mid-May to October/November): As the Mekong swells with monsoon rains, its water level rises significantly. This pressure forces the flow of the Sab River to reverse, pushing water from the Mekong northwestward into the Tonle Sap. This influx dramatically expands the lake's surface area and depth, transforming it into a massive floodplain reservoir.12
- Dry Season: As the Mekong’s water levels recede, the process reverses. Water drains from the Tonle Sap back through the Sab River and into the Mekong, returning the lake to its smaller, dry-season state.12
This seasonal pulse is vital to the region, as it makes the Tonle Sap one of the world’s most productive sources of freshwater fish and a designated World Network Biosphere Reserve.12