Records of river flows are available for various points along the Mekong mainstem and its tributaries. A historical overview of the main flow measurements is provided here. 

The Mekong flows are mainly dominated by to the south-west monsoon between June and October. This results in an annual flood pulse and therefore a distinct seasonality in the annual hydrological cycle between a flood season and a low flow season.

The upper Mekong in China contributes 16 percent of the total flow in an average year, while 55 percent comes from the large left bank tributaries in Lao PDR along with the Se Kong, Se San and Sre Pok (3S) river system. The estimated mean annual flow of the basin as a whole is almost 460 km³ , in an average year about 75 percent occurs within just four months between July and October.

In a large river system like the Mekong, seasonal flows can be quite variable from year to year. Figures on the right show average, maximum, and minimum monthly observed flows over 46 years (1960-2005) at key stations on the Mekong mainstream. The average flow volumes are presented in table below.

The seasonal cycling causes a large flow reversal to and from the Tonle Sap Lake in Cambodia. The floodplain in the lake extends over 15,000 km² and stores 50-80 km³ of water. The reverse flows normally begin in late May and end in late September. The Prek Kdam station is used to monitor flows in and out of the lake. Historical water levels at Prek Kdam in an average (2008), the wettest (2000), and the driest (1998) years between 1986 and 2008 is shown in the figure below.

 Average monthly flows from 1960-2005 (million m³)

   Observed water levels at Prek Kdam