Hand 2.1 The Mekong and Water Resource
1.
: Mekong stretch over 4000 kms from its head water in Tibet ( altitude 15,000 feet) to
South China sea. It comprises catchment area of 795,000 kms, one of the world largest river. The Mekong covers 6 countries; China, Myanmar ( Upper Mekong), Laos, Thailand, Cambodia and Vietnam (Lower
Mekong).
MRB comprises of the following geographical zones
•
Upper Mekong Basin.
The Mekong is called Lancang in China. It drains from Tibetan plateau at
4,970 meters down to Simao at 310 meter. It comprises of steep mountains and valleys with river falls 6.5 kms.
•
Lower Mekong Basin
1.
Northern Highlands . This part includes region of northern Loas and northern Thailand ( subarea 1 and 2 of BDP) eastward to Annamite Cordilla in Vietnam. Elevation peaks at 2800m.Topography is generally steep. The area has extensive Permian limestone which forms specular karst topography across Laos.
2.
Isaan ( Korat Plateau) and Central Laos: The Mekong cut deeply into the eastern rim of Korat plateau forming cliffs above the Mekong in places. Most of the plateau lies within Thailand, and has been extensively cleared for agriculture.
3.
Eastern Highland: The eastern highland is a south extension of the northern highlands. The mountains extend 700 kms from Laos through Vietnam. A number key large tributaries of LMB arise from this region including Se Kong, Se San and Sre Pok.
4.
Cambodian plain and Great Lake: The area is essentially flat and there are evidence of numerous change in river course. Large areas are flooded in the high flow period. Tonle Sap is the largest body of fresh water in Southeast Asia, and from one of the key features of the lowland.
5.
Delta : The Delta also contains unique and important ecosystems, including the wetlands of the Plain of
Reeds, and the mangrove forest in the southwest. The Delta is the rice bowl of the Basin.. An extensive network of canals has been developed, to provide irrigation water and transport.
. Water volume & discharge: In term of water volume, the Mekong has a annual discharge of 475 km3/year or 15,060 m3/second. China contributes 16% and Myanmar 2% and the remaining (82%) comes from the 4 lower Mekong countries. More than half of this amount comes from the mountain region along the eastern rim of the lower basin. This is due to higher rainfall in this area .
Description Yunan
China
Area-KM2 165,000
Myan mar
Laos
24,000 202,000
Thailand Cambodia Vietnam total
184,000 155,000 65,000 795,000
21 3 25 23 20 8 100% Catchment : as % of MRB
% of country
( state) area in LMB
Water contribution
%
Average flow m3/sec
21
16
2400
4
2
300
97
35
5270
36
18
2560
86
18
2860
20
1660 15,060
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Hand 2.1 The Mekong and Water Resource
Figure 1. About 55% of LMB water contribution comes from mountain area in the east of LMB which has higher rainfall
Pattern of discharge in the Mekong main stream is seasonal. Flooding peaks in August in the upper part of LMB and in September in the Lower part of LMB.
3 5 0 0 0
3 0 0 0 0
2 5 0 0 0
2 0 0 0 0
15 0 0 0
10 0 0 0
Figure 2 Mekong at Pakse.
Annual discharge (monthly flow, maximum and minimum)
4 5 0 0 0
4 0 0 0 0
T he M e k on g a t P a k s e , ma x imum, me a n , min imum f low ( 1 9 2 5 - 2 0 0 1 )
M EAN M ONTHLY
M ONTHLY M AXI M UM
M ONTHLY M I NI M UM
5 0 0 0
0
Jan Feb M ar Apr M ay Jun Jul Aug Sep Oct Nov Dec
The flow of 4 months of the year, from July to October, account for 64% of all annual flow at Chiang Saen
( Upper LMB) and for this same period it account for 74% of average annual flow in Phnom Penh ( lower
LMB) During this period, extensive floodplains are inundated covering all area below Khone fall to the sea.
The submerged in any year is estimated to be around 1-4 millions hectares.
Tonle Sap Lake of Cambodia and flood plain around Phnom Penh act as flood “ buffers” for the Vietnam delta. From May to October every year, as the water in the Mekong rises, it spreads across the floodplain and pushs water up Tonle Sap river into the Great lake up north. With this flow, the Great Lake expands from 2500 km2 in the dry season to 13,000 km2, and from water depth of 3 meters to 10 meters in the wet
2

Hand 2.1 The Mekong and Water Resource season. When the Mekong river falls later in October, much of the water flows off the flood plains, reverse of Tonse Sap lake to main stream down to Vietnam Delta.
4. Water Quality.
Chemical composition of the Mekong is similar to the mean of world river water. The flow from the upper Mekong shows influence of limestone, with high concentration of calcium. It is slightly diluted by large tributaries in northern & central Laos. Concentrations of nitrogen are below the world average, while concentration of phosphorus is relatively high. (phosphorus is associated with fine particle such as clay)
At present MRC monitor water quality once or twice per month at 98 sites.
Trend in water quality: 3 commonly identified water quality issues in LMB
Sediments in the water
Salinity ( NE Thai & VN Delta)
Eutrophication
The parameters used in monitoring these 3 concerns are TSS ( total suspend solids) Conductivity and
Total P and N ( phosphorus/nitrogen) respectively .
TSS at Chaeng Saen (1985-1992)
2500
2000
1500
1000
500
0
Feb-82 Nov-84 Aug-87 May-90 Jan-93 Oct-95 Jul-98 Apr-01 Jan-04
DATE
4.1 For sediment: Concern in the basin that deforestation and intensification of agriculture may lead to increased erosion. TSS trend data from last 15 years indicate decrease in most areas ( of total 23 sites) , increasing only at 6 sites. This doesn’t mean erosion is not a problem, but is probably due to sediment being trapped in storages
4.2 Conductivity ( for salinity) In NE Thailand where there has been concerns for salinity, 4 of 5 sites monitored show a decrease, and most concerned sites in Vietnam show also a decrease
4.3 Eutrophication: phosphorus and nitrogen are most common “nutrients limiting” algal growth
in aquatic ecosystem. If they present in great amount, algal bloom may result- and toxic to
aquatic life, and people. Trends in phosphorus and nitrogen, most sites outside the delta are found to be in good condition. In the delta, 40 sites in the delta show an increase in phosphorus and
31 site in delta show increase in nitrogen .
3