Natural Environment

The Mekong River arises on the Tibetan Plateau in the Thang Hla Mountains at approximately 5.100 meter above sea level (Kite, 2001). From the Himalayan Mountains it drains southward until it reaches the South China Sea. On its way it crosses the countries of China (Upper Mekong River /Lancang Jiang) as well as Myanmar, Lao PDR, Thailand, Cambodia and Vietnam (Lower Mekong River (LMR)), covering a catchment area of 795.000 km2 (MRC, 2005). On its first transect, the Mekong and its tributaries flow through narrow and steep gorges of which many of them are inaccessible. Therefore, the exact estimation of total river length is difficult. Estimates range from 4.100km to 4.900km river length, making the Mekong the largest and most important river in South East Asia (Johnson and Kummu, 2012; Kiem et al., 2008). The Upper Mekong River Area is characterized by tundra vegetation and montane, semi-desert ecosystems (Thompson, 2013), which limit any agricultural expansion on the steep slopes. Further downstream, the river leaves the Himalayan mountain area and most parts of the Lower Mekong River are surrounded by evergreen and deciduous forest (Ishidaira et al., 2008). Clear impacts of demographic and economic development like the expansion of agriculturally used areas and large deforested areas can be seen in this region (Thompson, 2013).

Main tributaries, which contribute to as much as one third of the total Mekong River discharge (MRC, 2005), are the Chi and Mun rivers draining into the Mekong from the eastern riverside as well as the Se Kong and Srepok draining the highlands of Vietnam on the western river side further downstream.
Regarding the mean annual discharge at the river delta of 475 km3/a, the Mekong River ranks 10th among the world largest rivers (MRC, 2005). However the discharge of the Mekong varies significantly with climate conditions and local precipitation events along its tributaries. The Upper Mekong River flow can be described as a nival runoff flow regime, depending on snow fall and snow melt on the Tibetan Plateau; whereas the LMR and its tributaries show a pluvial regime with clear high and low flow periods according to monsoonal events. Especially those last mentioned monsoonal rainfalls, which are dominated by the wet southwest and the dry northeast monsoon, lead to a complex hydrology of the Mekong basin. The annual, intra-annual and high spatial variability of weather conditions in the Mekong catchment generate locally very different runoff flows, since the weather systems are often not large enough to affect the whole basin (MRC, 2010a). The southwest monsoon (May-October) entails about 90% of the yearly rainfall, with mean annual precipitation rates between 1.000mm in northeast Thailand and 3500mm in Laos. From October to March the northeast monsoon causes dry climate conditions (Kite, 2001). Depending on these seasonal precipitation patterns the river level varies significantly throughout the year. Peak flows of about 45.000m3/s are reached during wet season in September/October (near Phnom Penh) whereas the Mekong River level drops down to 1.500m3/s in March/April during dry season (Kite, 2001). According to Kummu and Sarkkula (2008), it is important to maintain this annual flood pulse for the Mekong River ecosystem. Especially the Cambodian floodplain and the Mekong River Delta, some of the most productive ecosystems in the Mekong region, depend on the discharge and sediment yield from upstream. The Tonle Sap Lake ecosystem is likewise regulated by the natural hydrological patterns. While the Mekong River provides water for the lake system during wet season, the flow direction changes during dry season and the Tonle Sap drains into the Mekong, generating a unique natural phenomenon.
This brief description of the main hydrological and climate characteristics of the Mekong basin shows that they must be studied carefully since they are complex and above all highly important for the socio-economic development in the region. For example, flood and drought events depend on rainfall patterns which are spatially highly variable, as mentioned above. Therefore they hardly ever occur in the whole basin at the same time (Adamson & Bird, 2010) and must be addressed in their specific natural and socio-economic environment.

References and further reading:

Adamson, P. T., Rutherfurd, I. D., Peel, M. C. Conlan, I. A. (2009) The Hydrology of the Mekong River,The Mekong: Biophysical Environment of an International River Basin, Academic Press Elsevier, 53-76.

ADB (2011) Lower Mekong Basin Component Flood Vulnerability Indices,TA-7276-REG Supporting Investments inWater-Related Disaster Management, Draft final report Asian Development Bank, 1-59.

Beechham, R., Cross, H. (2005) Modelled Impacts of Scoping Development Scenarios in the Lower Mekong Basin, Mekong River Commission, Cited by: MRC (2009d), Hydrological and Flood Hazards in the Lower Mekong Basin, Mekong River Commission Secretariat, Vientiane, Lao PDR, 1-324.

Boucharel, J., Dewitte, D., du Penhoat P., Garel, B., Yeh, S.-W., Kug, J.-S. (2011) ENSO nonlinearity in a warming climate, Climate Dynamics 37: 2045–2065, DOI 10.1007/s00382-011-1119-9.

De Bruijn, K. M. (2005) Resilience and Flood Risk Management, A Systems Approach Applied to Lowland Rivers, Delft University Press, Delft, Netherlands.

Delgado, J. M., Merz, B., Apel, H. (2010) Flood trends and variability in the Mekong River, Hydrology and Earth System Sciences, 14, 407-418.

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FIVAS (2007) Ruined Rivers, Damaged Lives, The Impact of the Theun-Hinboun Hydropower Project on Downstreams Communities in Lao PDR, FIVAS, Oslo, Norway, pp 66.

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Halls, A. S., Burnhill, T. J., Kshatriya, M. (2009) Modelling the Cumulative Barrier and Passage Effects of Mainstream Hydropower Dams on Migratory Fish Populations in the Lower Mekong Basin, MRC Technical Paper No 25, Vientiane, p. 103.

Hannaford, J., Lloyd-Hughes, B., Keef, S., Parryand, C., Prudhomme (2011) Examining the large-scale spatial coherence of European drought using regional indicators of precipitation and streamflow deficit, Journal for Hydrological Processes 25, 1146–1162, DOI: 10.1002/hyp.7725.

Hoanh, C. T., Guttman, H., Droogers, P., Aerts, J. (2003) ADAPT: Water, climate, food and environment under climate change, The Mekong basin in Southeast Asia, International Water Management Institute, Mekong River Commission, Future Water, Institute of Environmental Studies. Colombo, Phnom-Penh, Wageningen.

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Ishidaira, H., Ishikawa Y., Funada S., Takeuchi K. (2008) Estimating the evolution of vegetation cover and its hydrological impact in the Mekong River Basin in the 21st Century, Hydrol. Process, 22, 1395–1405.

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