Mekong Delta's floods are
losing their vitality
The beneficial annual floods Vietnam’s Mekong Delta gets has been steadily dwindling for the last 14 years. The sediment flows have now dropped by 70% compared to before 2000, and many fish species have disappeared.
Formed some 6,000 years ago the Mekong Delta has long been a land nourished by floods. Each seasonal rise of the river brought silt and sediments, building the terrain, offsetting natural subsidence and replenishing fish and shrimp, the basis of local livelihoods.
But this natural nutrient source is diminishing, leaving the delta’s ecosystem increasingly fragile. The sediment flow has decreased to just 35–50 million tons a year (measured at Kratie, Cambodia). Land subsidence is worsening due to excessive groundwater extraction.
Some 17% of the world’s freshwater fish catch is in the Mekong River. But today, many large mammals, reptiles, birds, and fish, especially economically valuable species, have become locally extinct or extremely rare. Some 90% of people living in the Mekong Basin are farmers who depend on the river’s fertile silt and abundant aquatic resources. As these decline, their livelihoods are increasingly at risk.
The International Union for Conservation of Nature lists several Mekong Delta fish species as critically endangered:
Mekong giant catfish (Pangasianodon gigas)
Found across the Mekong Delta, lives in rivers, streams and year-round wetlands
Giant barb (Catlocarpio siamensis)
Inhabits the middle Mekong and Dong Nai basins; prefers floodplains and deep river pools
Jullien’s golden carp (Probarbus jullieni)
Found in deep, sandy sections of the Mekong
Giant pangasius (Pangasius sanitwongsei)
Lives in major forest-bordering rivers of the Mekong and Dong Nai systems
The annual “floating-water season,” or flood season, typically begins in July and lasts three to four months.
During this period farmers pause their fieldwork to fish and live in rhythm with the rising waters. Unlike the rapid and destructive flooding that occurs in central or northern Vietnam, floods in the Mekong Delta come and retreat slowly, bringing fertile silt and fish to sustain its 48,000-square-kilometer plain.
“In the dry season we walk on two feet; in the flood season we move with two hands and a boat,” a local man named Tuan says referring to how people in the Long Xuyen Quadrangle and Dong Thap Muoi regions have long lived with floods.
Over the past 50 years, however, the flood pattern has changed dramatically. Since 2012 the delta has seen no major inundation. In 2015 water levels at the Tan Chau Station in An Giang Province reached only 2.51 meters, the lowest ever recorded.
Along with the receding waters, both silt and the natural bounty that once came with floods have also declined sharply, disrupting the lives of more than 17.5 million people across the delta.
Flood levels at Tan Chau Station (An Giang) in the Mekong River upstream has been declining
Source: Southern Institute of Water Resources Research
“The Mekong’s floods are losing two of their most vital resources: Silt and fish,” Marc Goichot, a technical expert at WWF’s Asia-Pacific Freshwater Program, says.
He explains that the delta’s floods are gradually losing their connection with the river in three dimensions: Longitudinal (upstream to downstream), lateral (river to floodplain) and vertical (riverbed to groundwater). The main causes are hydropower dams and reservoirs, dikes and levees and sand mining, he says.
Stretching 4,880 kilometers from China’s Qinghai Plateau to the East Sea, the Mekong River, the world’s 12th longest, flows through six countries before reaching the sea in Vietnam’s Mekong Delta.
As floodwaters move downstream, they are increasingly trapped by a growing network of dams and reservoirs designed to store water and slow the flow. According to Vietnam’s Southern Institute of Water Resources Research, since 2010 the total storage capacity of upstream structures has reached 65–70 billion cubic meters, cutting the frequency of major floods in the Mekong Delta from once every four years to once every 15–20 years.
More than 400 hydropower dams are expected to operate along the 4,350-kilometer river in the coming decades, according to current estimates.
The Mekong River Commission’s 2023 “State of the Basin” report warns that these hydropower developments have significantly altered the river’s natural flow. During the flood season reservoirs store water, reducing flows, and in the dry season they release water to sustain power generation. These changes have disrupted the basin’s ecosystems and biodiversity.
Researchers at the Southern Institute of Water Resources Research project that by 2050, when all planned reservoirs with a combined capacity of about 110 billion cubic meters are completed, large floods will become rare, occurring only once every 25 to 50 years. Smaller floods, and even flood-free years, will be more common.
“This is particularly alarming because medium and large floods carry most of the sediments that sustain the delta,” Nguyen Phu Quynh, deputy director of the institute, says.
The total amount of sediment reaching the Mekong Delta is now only 35-50 million tons per year (measured at the Kratie station in Cambodia), a decrease of 60-70% compared to before 2000.
Sediments are a natural mixture of sand, silt, clay, and organic matter transported by flowing water.
When the current is strong, rivers carry more of them, but when water slows behind a dam, heavier particles settle, and finer sediments often get trapped upstream.
Hydropower dams have transformed the Mekong from a free-flowing river into a chain of reservoirs. This has severely reduced river biodiversity and blocked the migration of aquatic species, especially fish, between upstream and downstream habitats.
After flowing through a bunch of dams, floodwaters reach the Mekong Delta via two main branches, the Tien and Hau rivers, and pass through natural flood-retention areas such as the Long Xuyen Quadrangle and Dong Thap Muoi. These lowlands allow floodwaters to spread, settle and recede naturally.
To reduce flood damage and stabilize agriculture, local authorities have built extensive dike systems in these regions. Major rice-producing provinces such as Dong Thap, An Giang and Can Tho account for most of the delta’s dikes.
Dikes here fall into two main categories. Medium dikes protect the summer–autumn rice crop, typically in August, and high dikes block floods entirely and support triple-cropping. While these structures help farmers better control their planting schedules, they also prevent sediment from reaching farmlands, especially in flood-prone areas.
In natural conditions, floodwaters distribute sediments: coarse sand and gravel settle along riverbanks, while finer silt and mud reach the coast and help expand the delta’s shoreline. Today sediment levels are falling sharply across the region, made worse by illegal sand mining.
When sand is extracted from riverbeds, it leaves behind deep pits that trap incoming sediment. The river’s flow can no longer replenish the lost material, creating “hungry water”, a condition that increases erosion along riverbeds, banks and floodplains.
Experts argue that water management must take place at both regional and local levels. Within the delta, solutions must be tailored to specific ecological and economic zones. Cooperation is essential across borders.
Because the Mekong flows through multiple countries, hydropower projects or sand mining upstream can trigger severe impacts downstream.
Quynh of the Southern Institute of Water Resources Research stresses the need for a formal mechanism to share data, especially hydrology and reservoir operations, among Mekong nations. Such coordination, he says, will provide a scientific foundation for sustainable water governance.
Goichot calls for urgent action at the national level.
Domestic policies are easier to enforce, he notes, and cutting sand extraction should be a top priority to restore sediment flows and prevent further erosion.
From an infrastructure perspective, the Southern Institute of Water Resources Research recommends managing water use based on ecological zones aligned with key production systems: coastal aquaculture, fruit orchards and rice farms.
In areas where floods have weakened, farming models should adapt to new environmental and market conditions, it adds.
Associate Professor Le Anh Tuan of Can Tho University says closed-dike areas need a reevaluation of their agricultural strategies.
With less flooding and sediment, soil fertility has declined, pests have increased and farmers now rely more heavily on chemical inputs, he points out.
“Adaptation strategies must start with a mindset that values every form of water because water is the fundamental resource that will determine the sustainable future of the Mekong Delta.”
Writer: Phung Tien
Designers: Tam Thao, Minh Thu, Khanh Hoang, Thanh Ha
Reference sources
- Mekong River Commission
- Hydropower impacts on riverine biodiversity. 2024. Fengzhi He and partners.
- History of dykes in Vietnam. 2021. Vietnam Disaster and Dyke Management Authority.
- Impacts of dyke development on rice production in An Giang Province and flooding on the Mekong River system. 2013. Huynh Minh Thien and partners.
- Assessments of the accumulation and ingredients of sediments in and outside closed dykes in An Giang. 2017. Bui Thi Mai Phung and partners.
- Conference on establishing the Mekong Delta’s sank bank. 2022. Vietnam Disaster and Dyke Management Authority and WWF-Vietnam.
- Extent of illegal sand mining in the Mekong Delta. 2024. Yuen, K.W., Park, E., Tran, D.D. and partners.
- Sand mining in the Mekong Delta revisited – current scales of local sediment deficits. 2019. Jordan, C., Tiede, J., Lojek, O. and partners.
- Biodiversity in the Mekong Delta, Vietnam. 2024. Nguyen Duc Tu, Pham Thu Thuy, Tang Thi Kim Hong
- The Mekong’s Forgotten Fishes. 2024. WWF