Mekong
Water flow along its course

Table 1: Country share of Mekong River Basin (MRB) and water flows

By taking into account hydrological regimes, physiography land use, and existing, planned and potential resource developments, the Mekong is divided into six distinct reaches:

Reach 1: Lancang Jiang or Upper Mekong River in China. In this part of the river, the major source of water flowing into the river comes from melting snow on the Tibetan plateau. This volume of water is sometimes called the "Yunnan component" and plays an important role in the low-flow hydrology of the lower mainstream. Even as far downstream as Kratie, the Yunnan component makes up almost 30% of the average dry season flow. A major concern is that the ongoing and planned expansion of dams and reservoirs on the Mekong mainstream in Yunnan could have a significant effect on the low-flow regime of the lower Mekong basin system.

Reach 2: Chiang Saen to Vientiane and Nong Khai. This reach is almost entirely mountainous and covered with natural forest although there has been widespread slash and burn agriculture. Although this reach cannot be termed "unspoiled", the hydrological response is perhaps the most natural and undisturbed of all the lower basin. Many hydrological aspects of the lower basin start to change rapidly at the downstream boundary of this reach.

On 19 July 2019 this reach of the river dropped to its lowest level in a century. Officials are particularly concerned as July is in the wet season, when mainstream flows are abundant historically. Locals are blaming low water on the newly constructed Xayaburi Dam, as it enters its test phase prior to the start of commercial operation in October 2019.

Reach 3: Vientiane and Nong Khai to Pakse. The boundary between Reach 2 and 3 is where the Mekong hydrology starts to change. Reach 2 is dominated in both wet and dry seasons by the Yunnan Component. Reach 3 is increasingly influenced by contributions from the large left bank tributaries in Laos, namely the Nam Ngum, Nam Theun, Nam Hinboun, Se Bang Fai, Se Bang Hieng and Se Done Rivers. The Mun - Chi river system from the right bank in Thailand enters the mainstream within this reach.

Reach 4: Pakse to Kratie. The main hydrological contributions to the mainstream in this reach come from the Se Kong, Se San, and Sre Pok catchments. Together, these rivers make up the largest hydrological sub-component of the lower basin. Over 25% of the mean annual flow volume to the mainstream at Kratie comes from these three river basins. They are the key element in the hydrology of this part of the system, especially to the Tonle Sap flow reversal.

Reach 5: Kratie to Phnom Penh. This reach includes the hydraulic complexities of the Cambodian floodplain, the Tonle Sap and the Great Lake. By this stage, over 95% of the total flow has entered the Mekong system. The focus turns from hydrology and water discharge to the assessment of water level, over- bank storage and flooding and the hydrodynamics that determine the timing, duration and volume of the seasonal flow reversal into and out of the Great Lake.

Reach 6: Phnom Penh to the South China Sea. Here the mainstream divides into a complex and increasingly controlled and artificial system of branches and canals. Key features of flow behaviour are tidal influences and salt water intrusion. Every year, 35–50% of this reach is flooded during the rainy season. The impact of road embankments and similar infrastructure developments on the movement of this flood water is an increasingly important consequence of development.

Table 2 summarises the mean annual flows along the mainstream. The mean annual flow entering the lower Mekong from China is equivalent to a relatively modest 450 mm depth of runoff. Downstream of Vientiane this increases to over 600 mm as the principal left bank tributaries enter the mainstream, mainly the Nam Ngum and Nam Theun. The flow level falls again, even with the right bank entry of the Mun-Chi system from Thailand. Although the Mun–Chi basin drains 20% of the lower system, average annual runoff is only 250 mm. Runoff in the mainstream increases again with the entry from the left bank of the Se Kong from southern Laos and Se San and Sre Pok from Vietnam and Cambodia.

Table 2: Lower Mekong Mainstream annual flow (1960 to 2004) at selected sites.

Flows at Chiang Saen entering the lower basin from Yunnan make up about 15% of the wet season flow at Kratie. This rises to 40% during the dry season, even this far downstream. During the wet season, the proportion of average flow coming from Yunnan rapidly decreases downstream of Chiang Saen, from 70% to less than 20% at Kratie. The dry season contribution from Yunnan is much more significant. The major portion of the balance comes from Laos, which points to a major distinction in the low-flow hydrology of the river. One fraction comes from melting snow in China and Tibet and the rest from over-season catchment storage in the lower basin. This has implications for the occurrence of drought conditions. For example, if runoff from melting snow in any given year is very low, then flows upstream of Vientiane - Nong Khai would be lower.

In a large river system like the Mekong, seasonal flows can be quite variable from year to year. Although the pattern of the annual hydrograph is fairly predictable, its magnitude is not. The average monthly flows along the mainstream are listed in Table 3, providing an indication of their range and variability from year to year. At Pakse, for example, flood season flows during August would exceed 20,000 cubic metres per second nine years out of ten, but exceed 34,000 m /s only one year in ten.

Table 3: Mekong Mainstream monthly discharge, 1960–2004 (m /s).

There is little evidence from the last 45 years of data of any systematic changes in the hydrological regime of the Mekong.
detectable_format:json_format, language:response_language[Dutch]