The Tonle Sap River and Lake:

The Tonle Sap Lake is linked to the sea via the Tonle Sap River which converges with the massive Mekong River in Phnom Penh.  For five months of the year, this river undergoes a unique natural phenomenon.  It flows upstream.

During the rainy season (June to October) the massive Mekong River collects huge quantities of water.  Firstly, at its source in the Himalayas, summer time meltwater feeds the river and its many tributaries.  Secondly, as the river snakes through no less than five countries (China, Burma, Thailand, Laos and Cambodia) on its 4500 km long journey, it also accumulates the water from the heavy monsoon rainfalls along the way.

When the river finally reaches Cambodia’s capital it has swelled massively.  In Phnom Penh, the Mekong River converges with the Tonle Sap River in front of the Royal Palace.  Because the water volume of the Mekong is so large, it actually forces the current of the Tonle Sap River to reverse.  The backward water flow fills the massive Tonle Sap Lake further upstream. At the end of the monsoon season, in late October, the lake has swelled to an area of 13,000 km² and a depth of 10 metres.

At the start of the dry season, the Mekong River water levels drop while the Tonle Sap Lake water level is at its highest.  The large volumes of water of the lake push downstream forcing the Tonle Sap River to flow in its usual seaward direction.  This empties the lake again.  When the water is at its lowest level in May (just before the rainy season), the giant lake has retreated and only covers an area of 2500 km² at a depth of 2.2 metres. 

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