Updated documents on natural environment of The Mekong River Delta in Vietnam

Part Three: Surface water resources

3.1. Mekong River Basin
The Mekong River (the section through China is called the Lancang River, the section through Laos and Thailand is called Mae Nam Khong, the section through Cambodia is called the Mekong River and in the Mekong Delta is called the Cuulong River) is one of the rivers. The river is one of the the largest discharge rivers in the world with a total basin area of 795,000 km², a length of 4,909 km. The river basin is divided into: Upstream from the junction of Thailand - Laos - Myanmar border and up and downstream from this point to the East Sea. The entire Mekong basin area in Thailand, Laos, Cambodia and Vietnam is located downstream.

The Mekong River Basin in Vietnam covers an area of about 71,000 square kilometers, accounting for about 9% of the entire basin area and 20% of the total area of Vietnam. In the basin, there are small rivers located in Dienbien province in the Northwestern Region of Vietnam, then flowing to Laos, some rivers and streams in Quangtri province; the Secon River originates from Truongson mountain range, Aluoi district of Thuathien Hue province with a basin of nearly 3,000km², in which, the part on the territory of Vietnam is 750km²; the Sesan and Srepok rivers have basins located in 5 provinces of the Central Highlands of Vietnam.
In the Mekong Delta: Before entering Vietnam, the Mekong River is divided into two main branches, namely the Mekong River (on the territory of the Mekong Delta called the Tien River) and the Bassac river (on the territory of the Mekong Delta called the Hau river), discharged into the East Sea.
The area of the Mekong basin by countries and the percentage of contribution of water discharge by countries in the basin are as follows: China: 165,000 km², 16%; Myanmar: 24,000 km², 2%; Thailand: 184,000 km², 18%; Laos: 202,000 km², 35%; Cambodia: 155,000 km², 18%; Vietnam: 65,000 km², 11% (According to MRC, 2010 and Southern Institute of Irrigation Science, 11/2020). Thus, most of the surface water flow for economic development and daily life in the Mekong Delta of Vietnam is from abroad, partly from the Central Highlands. This is a big concern for the socio-economic development of the Mekong Delta.
3.2. The system of rivers and canals in the Mekong Delta
The Mekong Delta has the densest system of rivers and canals in the Mekong River Basin, including a system of natural rivers and canals.
3.2.1. The Tien river, Hau river and tributaries
The two largest rivers in the region are the Tien and Hau rivers. The Tien river is 257km long, from Tanchau at the border with Cambodia to the East Sea. The Tien river flows into the East Sea through 6 mouths: Tieu, Dai, Balai, Hamluong, Cochien and Cunghau. The Hau River is 258km long from Chau Doc at the border with Cambodia to the East Sea; 100km from the coast, it is divided into 2 large branches that pour into the East Sea through Dinhan and Trande mouths.
Although it is called "Cuulong: 9 dragons", in fact, for more than 100 years, the Mekong River has only 8 estuaries flowing into the East Sea as mentioned, in which the Balai estuary has a very small flow.

The Tien river at Tiengiang Province A branch of the Hau river at Cantho City

3.2.2. Other rivers originate from the border area with Cambodia
The most important of these rivers is the Vamco river system. This river has two major tributaries, Vamco Dong and Vamco Tay rivers, both originating from Prey Veng (Cambodia) and confluence in Tantru district in Long An province to form the Vamco river. The Vamco river pours water into the Soairap River, and then into the East Sea.
3.2.3. Rivers in the coastal area
The coastal area is the place where the rivers flow from upstream to the East Sea and the West Sea. From the Northeast to the Southeast, the main estuaries are: Soairap river, formed after the confluence between Vamco and Dongnai rivers. In Soctrang province, the Mythanh river flows from the inland into the East Sea; while close to the cape of Camau, the Bayhap and Cualon rivers connect the East Sea and the West Sea. Further north of Camau peninsula: Cailon and Caibe rivers in Kiengiang province are formed from small river systems inside the Mekong Delta and empty into the West Sea.
3.2.4. Inland major rivers and canals
The main rivers also receive water from many small inland rivers: Baodinh river and Gocong river (connecting with Tien river); Lapvo, Vamnao, Mangthit rivers (connecting Tien river with Hau river), Omon river, Cantho river connecting with Hau river. In addition, the Mekong Delta also has hundreds of large canals with a length of over 30km: Nguyenvan Tiep, Thapmuoi, Omon - Xano, Rachgia - Longxuyen, Rachsoi - Vamcong, Quanlo - Phunghiep, Vinhte, Triton - Bathe…and thousands of small canals divert water into the main streams. Currently, the canal system is dense at three levels: main canal, secondary canal, tertiary canal with a total of about 66,000km.
3.3. Hydrological mode
3.3.1. Overview
According to the data of the Mekong River Commission - MRC (2010), the total average annual discharge of the whole basin is about 460 billion m³. In Kratie: The average discharge in Kratie is 13,200 m³/s and the total annual discharge is 416 billion m³.
The hydrological regime in the Mekong Delta is directly affected by upstream flows, the East Sea tidal regime, part of the Gulf of Thailand tide, and is divided into 3 main regions:
          (i) The area is mainly affected by the upstream flood flow;
          (ii) Flood-tidal affected area;
          (iii) The main tidal influence zone.
The flow in the Mekong river basin fluctuates greatly in two seasons, which contrast sharply: The flood season accounts for about 90% of the total annual water volume; The dry season only accounts for about 10% of the total flow, the months from March to May usually the lowest flow occurs.
- Flood season with maximum flow of 38,000 - 40,000 m³/s, causing about 1.2 - 1.9 million ha inundation, depth of inundation from 0.5m - 4.5m. However, in recent years and in the future, large floods rarely happen.
- In dry season, the minimum flow is from 2,000 to 2,40 m³/s, so salinity of 4 g/L penetrates deep into the main stream from 30 to 70 km.
Natural climate cycles and fluctuations, especially El Niño events, lead to large annual flow fluctuations, with normal, rainy years (2000, 2001 and 2011) and dry years (1998, 2007 and 2016). Climate change, upstream dam construction and water use are severely affecting river flow regimes.
According to the documents of the Southern Institute of Water Resource Planning: During the period 1990-2015, the average annual flow of the river into the Mekong Delta, monitoring data at Tanchau and Chaudoc was 12,700 m³/s, of which through Tanchau is 10,109 m³/s and through Chaudoc is 2,575 m³/s. Documents from the National Center for Water Resources Planning and Investigation for the period 2000 - 2020 show that the average annual discharge measured in Tanchau and Chaudoc is 12,571 m³/s, of which in Tanchau is about 10,142 m m³/s and in Chaudoc about 2,430 m³/s. The total average annual amount of water entering the Mekong Delta at two stations Tanchau and Chaudoc is 398 billion m3 (12,620 m³/s), of which at Tanchau is 321 billion m³, Chaudoc is 77 billion m³. The total volume of water in the dry season (from December to May) is 94.3 billion m³ (accounting for 23.7% of the whole year).
In addition to very low freshwater flow from local sources (only 22 billion m³ per year due to rain endogenous), nearly 90% of water flow into the Mekong Delta is from upstream; The dry season is completely dependent on the upstream. Therefore, ecological resources and socio-economic development in the Mekong Delta depend greatly on water resources, especially from upstream. From there, it can be determined that water is an environmental issue that needs special attention in regional development planning and protection and rational use.
3.3.2. Fluctuations in average flow in the period 2014 - 2018
Detailed data on 5 consecutive years (2014 - 2018) on the average monthly flow of the Tien river (at Mythuan Hydrological Station, longitude: 105^o51^’, latitude: 10^o16^’) and Hau river (at Cantho Hydrological Station, longitude: 105^o47^’, latitude: 10^o00') issued by the Center for Hydrometeorological Information and Data of the General Department of Meteorology. From the data, it can be commented that the flow of Tien and Hau rivers in the 5 years of 2014 - 2018 is as follows:
a. The Tien river at Mythuan Station:
- Average discharge (Q+) in 2014: 13,467 m³/s; 2015: 12,292 m³/s; 2016: 13,008 m³/s; 2017: 14,833 m³/s and 2018: 14,500 m³/s, showing that in the 2 years of 2017 and 2018: the water flow downstream is higher than the previous 3 years.
- Monthly average (Q+) discharge: Usually high in the months of the flood season (August-October) and low at the end of the rainy season: the highest is 22,500 m³/s (September 2018) and the lowest is 10,800 m³/s (April 2014).
- The highest discharge (Q+) (max) in each year is: 25,500 m³/s (August 2014); 21,300 m³/s (September 2015); 22,500 m³/s (October 2016); 25,100 m³/s (August 2017) and 28,400 m³/s (October 2018).
b. The Hau river at Cantho Station:
- Average discharge (Q+) in 2014: 13,575 m³/s; 2015: 12,735 m³/s; 2016: 13,375 m³/s; 2017: 13,925 m³/s and 2018: 14,533 m³/s, showing that from 2014 to 2018: Water flow to the downstream shows signs of increasing, although not large.
- Monthly average (Q+) discharge: Usually high in the months of the flood season (8-10) and low at the end of the rainy season: The highest is 19,300 m³/s (September 2018) and the lowest is 10,100 m³/s (May 2015).
- The highest (Q+) discharge (max) in each year is: 20,200 m³/s (August 2014); 20/20 m³/s (August 2015); 20,700 m³/s (October 2016; 21,000 m³/s (September 2017) and 24,500 m³/s (September 2018).
Comparison of the flow of the Tien and Hau rivers:
- In general, the average flow (Q+) from upstream to Mythuan and Cantho stations is equivalent.
- The highest discharge (Q+) in each year in Mythuan (the Tien river) is higher than in Cantho (the Hau river) but not much.
- The time of highest discharge ime (Q+) between the two stations is in August - October.
3.4. Flood and inundation
Every year, the flood season in the Mekong Delta starts from June and July and lasts until November and December. Floods rise and fall slowly, quite gently, causing flooding in the North of the Mekong Delta on an area of about 1.2 - 1.4 million hectares (35% of the Mekong Delta area) in the low flood year and 1.7 - 1.9 million hectares (48% of the Mekong Delta area) in the major flood year, with a depth of 0.5 - 4.5m and a duration from 3 - 6 months. Fluctuations in both time and flood peak are not large from year to year. Since the Mekong Delta is very flat, the floods were larger than normal, causing prolonged flooding.
However, according to the latest document of the Southern Institute of Irrigation Science (November 2020): In the coming time in the Mekong Delta, floods will decrease more and will rarely appear large floods. In other regions in Vietnam, floods are natural disasters, but in the Mekong Delta, floods are a great source of benefits: Supply of fresh water; provide silt; provide aquatic resources; insecticides; maintain wetland ecosystems. Therefore, it is necessary to review policies and projects on flood drainage and to develop plans and works to store fresh water in the flood season for the whole region. Flooding and appropriate flood control is one of the important issues in water resource management in the Mekong Delta.
3.5. Drought
The Global Climate Risk Index in 2010 and 2019 shows that in the period 2015 - 2016, there were droughts affecting many areas in the territory of Vietnam and especially in the Mekong Delta. Along with drought, serious saltwater intrusion inland. Drought and saltwater intrusion have caused crop failure and affected people's lives. Drought and saltwater intrusion in 2020 will be more severe than previous years. Therefore, drought (and saltwater intrusion) are environmental issues that need special attention in the Mekong Delta Integrated Regional Planning.
3.6. Salinization
Due to the low, flat terrain and high tidal influence in the months of the dry season with low river flow, salinity intrusion penetrates deep into the estuary and into the inland canals of the Mekong Delta. This is the area with the largest and most serious saltwater intrusion in the country. Saltwater intrusion is increasing due to increasing impacts of climate change and sea level rise. The salinity boundary is 4 parts per thousand (g/L) inland to 50 - 55km along the Tien river and 40 - 45km along the Hau river; Most rivers and canals in Camau peninsula have year-round salinity of over 10 parts per thousand.
In recent years, salinity intrusion in the dry season 2015-2016 is the highest. However, in 2020, saltwater intrusion increased even more: The total flow to the Mekong Delta through two hydrological stations Tanchau and Chaudoc was reduced by 13% compared to the average of many years and lower than the total flow in February 2016, about 5%, therefore, saltwater intrusion in March 2020 increased sharply.
Detailed tables of hydrology of major rivers in the last 5 years and maps of floods, droughts, and saline intrusion are not provided in this summary.

Collection and analysis of updated data by VESDEC for Strategic Environmental Assessment of the Mekong Delta Integrated Regional Planning for the period of 2021 - 2030, with a vision to 2050.
November 2020.