2. Classification of topography in the Mekong Delta

According to a paper published by Mr. Nguyen Huu Chiem⁵⁾ (Chief professor of Department of Natural Resources Management, Collage of Agriculture, Can Tho University, Viet Nam), topographical model in the Mekong Delta could be divide into five major part that include lower sub-divisions and this classification could be supported Landsat pictures, topographical methods, characteristics of sedimentation, agricultural ecosystem and so on. The table and chart below is revised one based on his original paper.

Table- The topographical classification of the Mekong Delta by Dr. Nguyen Huu Chiem

Landform	Sub-units (A)	Sub-units (B)
Floodplain	High floodplain	Natural levee
		Sand bar
		Back swamp
		Closed floodplain
		Open floodplain
	Tide affected floodplain	Natural levee
		Back swamp
		Broad depression floodplain
Coastal complex	Sand ridge
	Coastal flat
	Inter ridge
	Mangrove swamp
Broad depression	Broad depression

Figure- Image of topographical disribution according to abouve division

Revised from "Geo-Pedological Study of the Mekong Delta", Nguyen Huu Chiem⁵⁾

On condition that we started to see the status from the sea, Mangrove swamp distribute along the coastal area face to South China Sea and we could see Coastal complex, which include some kinds of micro-landform like coastal flat around the two main river. Broad depression over Cape Ca Mau includes Peat depression is barren land. These large landforms correspond with limit line of saline intrusion in dry season, and this front line also shows some differences regarding agricultural form or land use. Inner areas are strongly influenced by fresh water from the Mekong River system. The nearest zone to the two main river is Back swamp followed by two kinds of flood plain at outer side. The uppermost area of main river have Natural levee and northern Cambodian border area consists of relatively old alluvial terrace. According to Mr. Chiem's explanation, each landforms have each characteristics like below;

(1) Floodplain
High floodplain has been submerged annually in the depth of 2-3m and most areas locate in northwest of Delta. This part includes some sub-divided landform like below. Natural levees are parallel Hau River and Tieng River and this landform occupies highest part of the Mekong Delta (3-4m to sealevel). The range of natural levees within 500m from the river and decrease its height gradually as distance from the river. Some natural levees also could be submerged only in high water season, September or October. Sandbars are surrounded by branches and the shape is very similar to natural levees on the Landsat pictures. But the materials or texture of deep part of soil are more coarse than natural levees. Back swamps are behind natural levees and vulnerable to flood. Some parts of this landform have 2-3m depth of flood water in its maximum on September and also have some kinds of acid-sulfate soils in it.

Besides these landforms, floodplain has pretty large area of wetland. Closed floodplain means Plain of Reeds surrounded by Sand ridge and main part locates at left bank of Tieng River especially at Dong Thap Muoi. In flood season, this closed floodplain become a large lake and its water depth record 3m in maximum. It is difficult to drain the water through East Vam Co River or My Tho River (Tieng River). In such hard conditions, about 60% of the land has been pioneered and the rest part is covered with Melaleuca forest or Eleocharis, which have strong tolerance against acid-sulfate soil.

This Plain of Reeds accord with "Plaine des Jones", which mentioned in Mr. Hori's book¹⁾. Inlet flood water of this plain come from Cambodia directly across the border in high water season. And in his book, East Vam Co River is called Vaico Oriental while Vietnamese name is Vam Co Dong (it means East Vam Co). Vam Co River system has two major branches, East Vam Co River and West Vam Co River. This river system is outside of the Mekong River but the border is not so clear especially in flood season. In addition to these natural property, the plain is also known as a warm corner of Vietnamese War because the tall reeds plants is favorable for hiding. Therefore U.S. army tried to drain the water by constructing canals and then sprayed herbicides⁶⁾.

Open floodplain is very different from Closed floodplain on the point of easiness of drainage because the area has gentle slope from Hau River to Thailand Bay. The maximum water level is in the range of 1.5-2.0m and also could find strong level of acid-sulfate soil.Tide affected floodplain is influenced by tidal water mainly through the two main river and it can be submerged 0.5-1.0m depth in its maximum season. The area still has fresh water all through the year. Some parts have acid-sulfate soil but its disadvantage is mitigated with flashing by tidal movement so the potential of agriculture is high.

(2) Coastal Complex
This area is affected by both marine and riverine conditions and its landforms could be divided into four sub-divisions. Sand ridges are created along the coastal line and have highest sealevel (2-5m) in coastal complex. The scale of sand ridges are in range of 0.5-2km in width and 5-40km in length. Costal flats have 1.0-1.5m height against sealevel and never be submerged directly by seawater. In dry season, however, saline water is lifted up to land surface due to capillary action. Inter ridges locate between sand ridges and its scale is small. The water movement depends on tidal action and this region is widely submerged from April to November with saline water. Acid-sulfate soil or potential acid-sulfate soil is very common condition in this region but tidal flashing and freshwater supply mitigate its disadvantage for agriculture. But the flooded period caused by both freshwater and saline water is too long. Mangrove swamps is a dominant landform of coastal line, estuaries and most part of Ca Mau Province. Potential acid-sulfate soil is common and top soil or subsurface soil are sulfated. The main species of mangrove forest are Avicennia and Rhizophora.

(3) Broad Depressions
This landform covers large area of southern Delta and enclosed with Can Tho Province or Kiên Giang Province. The shape of surface is very low (0.5-1.0 sealevel) and flat. By its topography, soil, vegetation etc., broad depressions are divided into two parts. Broad depression is isolated from the Mekong River System and water from Hau River cannot flow into this area. As a result, soil can be affected by saline water and acidification mainly in dry season. Peat depressions are two lowest part of broad depression which locate at U Minh Thuong and U Minh Ha in Ca Mau Province. The soil which is covered with Phragmites and Melaleuca forests has much peat. This area has annual rainfall of 3,000mm therefore also easy to be submerge. This depression become like natural lakes and supply irrigation water.

(4) The others
Old alluvial terrace locates northeast Delta across the Cambodian border. The area is about 150,000 hectare. Its soil is well pressed and includes many acidified sands. Hills and mountains are isolated from general landform of Delta and mainly locate at western area. The maximum point is 710m (Mount Cam). All the hills and mountains are granite except Kien Luong and Ha Tien district with limestone.

3. Coastal area and currents

The coastal sea of around the Mekong Delta is relatively shallow for a long distance from shore comparing to north Viet Nam. The area of under 20m in depth on the figure is revised from a map published in Viet Nam⁷⁾. This area is extended to about 30km out from coastal line of Bac Lieu and Ca Mau Province and Thailand bay has similar condition. More than half of this area might satisfy the definition of wetland by Ramsar Wetland Convention, at least 6m depth on the condition of ebb tide. But most of the areas are just the sea and the area of aquatic vegetation like mangrove forests are very limited.

Continental shelf, less than 200m depth, expands 200km out in maximum in the direction of south from the Mekong Delta and this area is most shallow coastal sea around the Indo-china peninsula. A report complied by UNDP in 1993 said, from 80% to 90% of fishery in Viet Nam come from coastal sea⁸⁾.

There are some currents around Delta. Most predominant one is a cold current from north to south. This current is related to the monsoon which has north wind ingredient in dry season or January⁹⁾. These currents originate from west coast of Philippine or down current from west coast of Taiwan isl. And there is also warm current from south to north but there is not enough information about it. But this might also have some relationships to the monsoon, especially in rainy season.