Handout 2.1 The Mekong and Water Resources
The Mekong and Water Resources
1. Physical feature: Mekong stretch over 4000 kames from its head water in Tibet (altitude 15,000
feet) to South China Sea. It comprises catchment area of 795,000 kames, 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 kames.
• Lower Mekong Basin
1. Northern Highlands. This part includes region of northern Laos 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 km from Laos through Vietnam. A number of 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 evidences of numerous
changes 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.
2. 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.
Table 1. Area of the 6 Mekong countries in the basin and their water contribution
Description
Yunnan
China
Myanmar
Laos
Thailand
Cambodia
Vietnam
Total
Area-KM2
Catchment: as % of
MRB
% of country (state)
area in LMB
Water
contribution %
Average flow
m3/sec
165,000
21
24,000
3
202,000
25
184,000
23
155,000
20
65,000
8
795,000
100%
21
4
97
36
86
20
16
2
35
18
18
2400
300
5270
2560
2860
1660
15,060
1
Figure 1. About 55% of LMB water contribution comes from mountain area in the east of LMB
which has higher rainfall
3. Water regime- Seasonal run of pattern
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.
Figure 2 Mekong at Pakse. Annual discharge (monthly flow, maximum and minimum)
45000
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 )
40000
35000
30000
25000
MEAN MONTHLY
20000
MONTHLY MAXI MUM
MONTHLY MI NI MUM
15 0 0 0
10 0 0 0
5000
0
Jan
Feb
Mar
Apr
May
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 pushes 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
2
meters to 10 meters in the wet 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
TSS (mg/L)
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 sites in delta show
increase in nitrogen.
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