Mekong river information and relevant websites

This report represents information of one of the largest river in Asia, Mekong River. It is also one of two important system rivers in Vietnam. Besides, the report is also introduced some relevant websites as well as some organizations which related with the Mekong River.

The Mekong Delta is a mega-delta with the area of 49,520km2, located at the end of the Mekong River This wetland which has complicated rivers and cannals are well known as the world's storage of granary, especially of rice The land is generally thought

as the fertile soil, however, most of the areas have sever problem for farming due to delta's geological history Actually, the area was once under the sea and the influence still remains in the soil as Acid Sulphate Soil Delta's flat topography has boosted highly land-use, and the population density is also high The Mekong Delta is now facing to many urgent problems for sustainable development such as decrease of Mangrove forests, annual flood in the rainy season and saline intrusion at coastal area, and etc

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Mekong Delta in Viet Nam - http://cantho.cool.ne.jp

Rivers in the Mekong Delta

Structure and characteristics of rivers

Contents

1 River system in Mekong Delta

2 Longitudinal Section of Two Main River

3 Erosion (Pleae wait a while)

1 River system in Mekong Delta

As the former chapter, Lower Mekong Delta has many branches originated from two main stream of Hau River and Tieng River These streams have carved by the tractive force of natural water flow In addition, there are huge number of canals that have been constructed with purposes of irrigation, drainage, water-way, and guerilla sweeping This canal system has developed quite well and almost all the place in

Mekong Delta might be connected Here, however, the system means natural river The below chart is the location map of major rivers and its estuary in Mekong Delta based on some maps published in Viet Nam4) 5) followed by a explantion according a report6)

Fig Major rivers and estuaries in Mekong Delta

The Hau River (Bassac River) come from Cambodia to Viet Nam and keeps its name until the South China Sea The Hau River meanders again and again from Phnom Penh to An Giang Province, Viet Nam, and the main flow follows a fault whose direction is the Northwest to the Southeast The Hau River flows on a parallel with the Tien River which has larger scale than the Hau River From Chau Doc to the South China Sea, the Hau River has completely straight shape because of a fault After passing the Vam Nao Channel (or River), which connects the two main river at Cho Moi, the unbalance of the flow are dissolved, and the river width becomes expanded Besides the channel, there are uncountable natural and artificial canals network between the two main rivers The Hau River has two estuaries, Dinh An and Tran De, while Bat Sac, no longer was also an estuary of Hau River and

is still one of the nine heads of the Mekong Delta

In the mean time, the Tien River (Mekong River) also comes into the border of An Giang Province and Dong Thap Province from Cambodia territory From the uppermost delta to vicinity of My Tho City, the river shows bow-like shape due to a fault which is an imprint of geological joint movement Each branch at lower Mekong Delta has been influenced with the fault followed by Hau River and another faults run from Ca Mau to Tan An which has a direction from the Northeast to the Southwest Thus, the Tien River has complicated shape

The section from Ben Dinh / Tan Chau to the north of My Thuan, the Tien River meanders and repeats divisions and concourses; therefore there are so many sandbars or islands within the river The cause of this complicated shape is attributed to faults From Ben Dinh to Long Thanh, the Tien River flows

according to the fault and the riverbanks in this section are severely eroded From Long Thanh to the south of Cao Lang, the river meanders widely and defined three major faults Near Phu Loi, the river was separated to three flows with Cai Vung Island and Long Khanh Island (refer the figure), and the

northernmost flow are largely bent by two faults From Phu Binh to Cho Moi, the Tien River is divided into two flows by Tay Island The Northeast flow follows a Nortwest-Southeast fault and has a straight shape

On the other hand, the Southwest flow provides water from the Tien River to the Hau River through the Vam Nao Channel which is on a Northeast-Southewst fault

The Tien River also has two flows in the section of Cho Moi-Con Lan The west flow follows a fault

originally located from Hong Ngu to Cao Lan and has straight shape, while the east flow has bow-like shape From Con Lan to the Southeast of Cao Lanh, Tien River is divided into four flows and has Chai Island and Tre Island The river in this segment has had a tendency to move toward the South, and the oldest flow is Cao Lanh Canal From here to Binh Hung, the river is single stream and has a movement to the North From Binh Hung to the North of My Thuan, the river has two main streams and has a

movement toward the Southwest The area just upper side of Sa Dec are heavily eroded and the area has been expended to upper 200m against the city From the point which is about 15km west to Vinh Long, the Tien River breeds many canals and the river is divided into the Co Chien River and the My Tho River (or Tien River) at Vĩnh Long City The under picture was taken at My Thuan Bridge near Vinh Long City and shows separation point of Co Chien River from Tien River

Pic Separated point of Co Chien River from Tieng River at My Thuan Bridge, Jan 2003

After that Tieng River produces Ham Long River and Ba Lai River which enclose Ben Tre Province (like

an island) Tieng River itself change its name to My Tho River at My Tho City although local people still call the river is Tieng Giang (Tieng River) And in the end, Tieng River subdivided to Dai River and Tieu River which mean Big River and Small River respectively and flow into the South China Sea

The chart above also has nine mouth corresponds to heads of the dragon The information form the Museum of 9th army at Can Tho City and the dragon legend based on real nine estuaries However, as mentioned above, Cua Bat Xac and Cua Ba Lai no longer show clear figure of mouth because of long-term sedimentation, therefore Mekong River has seven big estuaries at the end of the flow in reality But the Vietnamese lucky number of nine still be used to describe the delta The head word of each estuary,

"Cua" means "door" so could be translated to "mouth" of the river The under picture was taken at the Cung Hau Mouth, the point where Co Chien River pour into the South China Sea

Pic Cung Hau estuary, the mouth of Co Chien River at Long Hoa, Tra Vinh Province, Feb 2003

Thus when we overview the river system without artificial canals, Mekong River could be divided to main stream Hau River has single straight shape and provides water mainly to the west side of Delta Tieng River has some major branches and act as a main water pipeline to east side of Delta You can see some pictures of rivers in Mekong Delta

2 Longitudinal Section of Two Main River

So far regular systematic survey of each river in Mekong Delta hasn't been carried out There is an vertical shape of two main river although the detail of survey is not clear2) and you can see the image of this with revised chart The original chart has no data (value) and is described with plain and vertical section like this According to original figure, the survey seems to be done each 1-2 km span The simplified chart here shows a

hypothetical bed slope which is based on average depth in each 40km span

First, the Tieng River's inlet for Viet Nam from Cambodia is about 240km upper from its mouth, and this uppermost segment has near 40m in maximum depth and the segment is also deepest one through the whole Tieng River on the point of average depth From uppermost segment to just upstream point of My Tho City, water depth fluctuate within a range of 5-20m and decrease its average depth gradually The point 80km upper side from the sea record some 30m depth and from this point to the sea, most section has 10m

or below depth When the river reaches to its mouth, the depth becomes only 3m or thereabout

On the other hand, Hau River has about 200km length in Mekong Delta From the uppermost point, Châu ÐôLc City to Long Xuyên City, the

water depth repeats wide fluctuation from 5 to 30m and it turns shallow gradually Near Can Tho City, which locate on 100km point far from the sea is the deepest section but Tieng River is more deeper And then the river bed is lifted as the river going down and finally, there is 5m depth at the estuary

Consequently Tieng River lose its depth at downstream section due to separation of branches while its upstream section have relatively rich water depth In reverse, upper section of Hau River is not so deep and wide but its middle and downstream section has more depth compared with Tieng River

This figure from unclear original has not enough confidence for official use but this still shows us relative shape of two main river in Mekong Delta And we also have to consider tidal affection which has some 4m difference between high and low tide or seasonal difference between rainy season and dry season Mr Hori mentioned an interesting story about My Thuan Bridge which have constructed over Tieng River in his book1) The bridge constructed by Australian ODA after long term investigation is a part of National Road No.1 and this connects Tieng Giang Province and Vinh Long Province This bridge was planned with extremely high clearance because Cambodian Government inisted on the importance of navigation for large vessels from the sea to Phnom Penh But actually, estuaries of Tieng River has only 2m in dry season or 3m in rainy season, therefore that argument has no persuasion in reality The most

recommended route is combination of two main river, from Hau River via connection canal at An Giang Province to Tieng River and the route supports characteristics of two main river on the point of water depth My thuan Bridge gives no effect to this real route to inland

 References

1) Tap ban do hanh chinh Viet Nam (Administrative Atlas)CNha Xuat Ban Ban Do (Cartographic Publishing House)C2002

2) Tap ban do hanh chinh Viet NamCNha Xuat Ban Ban Do (Cartographic Publishing House)C2002 3) Mot so dac diem dia mao hinh thai song Tien, song Hau, Ha Quang Hai, Dia ly-Dia chat-Moi truong, Hoi dia ly - dia chat TP HCM, 12/1996

- Some physiographical characteristics of Tien River and Hau River, Union of Scientific & Technological Associations of Ho Chi Minh City 4) Nuoc Duoi Dat Dong Bang Nam Bo (Groundwater of South Plain), Ministry of Industry, Ha Noi, 1998

5) THE MEKONG: The Development and Its Environmental Effects, HORI Hiroshi, KOKON-SHOIN

Publishing, Japan, 1996

© AKIRA Yamashita, Department of Environment and Natural Resources Management, Can Tho

University, Vietnam

Web site: Mekong Delta in Viet Nam (http://cantho.cool.ne.jp)

E-mail: songcuulong@hotmail.com

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Flood and saline intrusion in the Mekong Delta

>>> Saline intrusion <<<

The water related natural disasters in the Mekong Delta can

be divided into "flood (or submergence)" and "saline intrusion" roughly The word,

"flood" means a phenomenon that excess river waters originated from upper stream submerege the area of inner dike in general However under the condition of the strong didal influence toward inland rivers and canals like the Mekong Delta, the flood tide also could cause a flood Therefore both of flood waters from fresh water basin and saline intrusion make the same output of "flood"

to the society, whereas the mechanism of each natural phenomenon are completely different As an essential major premise, the saline intrusion is very characteristic and important phenomenon to understand the natural status in the Mekong Delta The following figure from a material1) shows the range of saline intrusion The coastal sea of the Mekong Delta, the South China Sea, is well known with its large tidal difference in seawater level, and the differential exceeds 4.0m in maximum Under the condition that the most area of the Mekong Delta has only several meter in the height above the sea level, this large tidal movement has great influence to inland area, especially in the rainy season when the river waters from upper stream also increase While the change of water level due to tidal movement extend to Kompong Cham in Cambodia which is located at the upper 410km point from the sea along the main rivers, the intrusion

of seawater itself can reach about 60km far from the sea2) In the meantime,

Thailand Bay that face to the west coast of the Mekong Delta has only less than 2.0m in its tidal differential, therefore, the waters from the South China Sea thuroug the rivers and inland canals easily flow into Thailand Bay (from a specialist in

irrigation teaching at a collage in Can Tho City) Moreover, by the results from a tidal table of this area in 19863), Phu An in Ho Chi Minh City which faces to the South China Sea recorded 3.7m in maximum diffrential in a year (maximum

sealevel of 4.8m, minimum of 1.1m), while Ha Tien observatory shows only 1.4m in annual maximum diffrential (maximum sealevel of 1.7m, minimum of 0.3m)

At the above figure, the area intruded by seawater all the year around is the coastal edge from the estuary of Dong Nai River (Can Gio District which is well known as a UNESCO's natural reserved are) to Ha Tien near the Cambodian boder Particularly

in the Ca Mau peninsular, the area over 30km from the coastal line are influenced with saline, and it is very hard to produce agricultural crops Meanwhile, the three inner belts are seasonally intruded by saline, and each degrees depends on the distance from upper side or main streams It is clear that the upper zone shows lower saline concentration and shorter period of intrusion than the lower or oastal zone In the original reference, writer uses a Vietnamese word which correspond to

"improve" in English as an indicator to classify the delta into this three outer zone, and it might represent the possibility of land improvement for the farming in the

Mekong Delta The last one, which enclose both the upper area from Vinh Long Province and Can Tho Province and the area within about 15km from the main streams, is the area which can be provided freshwater all the year around This area is the most favorable land for agriculture on the point of water resources, and the major problem of Acid-sulfate soil is not so sever due to

continuous sedimentation from upper freshwater basin

　 The next figure1) shows several equivalent lines of saline concentration per a liter The lines

in another material published in

1974, which is quoted in Mr Hori's book2), are drawed at nearer side

to the sea, and this material also describes that the density turns lower, as the discharge of the Mekong River become larger That

is to say, riverine freshwater and mrine saline make a front-line at the inland of the Mekong Delta According to Mr Hori, the latitudes of saline concentration for irrigation rice crop are 1-2g/liter at planting season, 5-6g/liter at growth period, and 4g/liter and less at before harvest Therefore, it is easy to assume that the saline condition in the Mekong Delta is not

so advantageous The water volume at each mouth of tributary is very different each other, therefore, the distance of saline intrusion at each tributary also shows variations The distance from the sea which shows the saline concentration of 1.0g/liter is generally defined as 30km at the Hau River, 70km at the My Tho River and the Ba Lai River, 60km at the Co Chien River and 65km at the Ham Luong River1) These variations at each tributary are also shown in the right-hand figure This situation represents a tendency in the dry season when the tributaries are vulnerable to be intruded by saline In the rainy season, on the other hand, the front-line of fresh water and saline at each tributary is pushed down to the upper about 15km point from the coastal line2) In addition, at the vicinity of Ho Chi Minh City, surface water also include high salt in itself

On the viewpoint of agricultural production, even in the area which is traversed by the line of 10g/l in above figure like My Tho or Ben Tre, relatively matured irrigation system has enabled local residents to harvest rice crop twice in a year at least It is too difficult to prevent saline intrusion completely under the condition that

numerous rivers and canals connect to the sea like the Mekong Delta, while some delta regions in developed countries like the Mississippi Delta in U.S are well

managed with perfect dikes and providing appropriate amount of sedimentation to the back marsh

>>> Floods and inundations in the Mekong Delta <<<

The flood in the Mekong Delta occurs during the rainy season, from May to

December, and its peak is in September or October This natural event is annual one, and its phenomenon is quite different from the flood whose response against the rainfall is very fast like flash in a mountainous area Vietnamese language approximately has two words, "lu" and "lut", to express the flood First one, The word "lu" originally means a status of beeing risen water level in a river, while "lut" indicates damages due to the inundation4) Generally, Vietnamese called a flood "lu lut" And in the Mekong Delta, the mechanism of flood generation is different like the next table

The classification of flood in the Mekong Delta