The Mekong River is an exceptionally complex system with highly variable inter- and intra-annual flows.

Most of the total annual flow volume is delivered to the Mekong River from tributaries in the Lower Mekong River Basin, while the upstream flow contributes a smaller portion. However, the importance of upstream flow should not be underestimated as dry-season snowmelt from China contributes over 24% of the total annual flow.

Table 1. Proportional contributions to total Mekong River mean annual flow by river reach, distinguishing those made by the left and right bank tributary systems.

River Reach Left Bank (%) Right Bank (%) Total (%)
China 16 16
China – Chiang Saen 1 3 4
Chiang Saen – Luang Prabang 6 2 8
Luang Prabang – Vientiane 1 2 3
Vientiane – Nakhon Phanom 18 4 22
Nakhon Phanom – Mukdahan 3 1 4
Mukdahan – Pakse 4 6 10
Pakse – Kratie 22 2 24
Tonle Sap 9 9
Total 55 20 100


The flood season in the Mekong River Basin lasts from June to November and accounts for approximately 70-80% of the total annual flow. The annual flood season is especially important in the Lower Mekong River Basin, where it has shaped the environment and its inhabitants.

See the sections on the People and Flood and Drought for more information.

Many of the Mekong’s key ecosystems have developed as a result of seasonal flow variations. The area’s extensive wetland habitats would not exist without the annual flood. Likewise, the life cycles of many Mekong fish species depend on it. Fish migrate to deep pools in the mainstream to seek refuge during the dry season. During the flood season, they migrate back to spawning and nutrient-rich feeding grounds on floodplains.