Environment - September 4, 2023 | 03:00 pm PT

Vietnam's Mekong Delta, world's youngest river basin, might be first one gone

Nguyen Van Thom is proud to say that he is sensitive to all the changes in the Mekong, and is sad to say that the river "has lost its rhythm" for the last 20 years.

Thom’s family lived for years near Cambodia’s Tonle Sap Lake before moving downriver to settle in An Giang Province in southern Vietnam.

When his Cambodian neighbors complained about a drop in fish stocks in 2019 and that the lake’s water was no longer brownish but had turned blue instead, he knew it was bad news since it meant there was little alluvium in the water, and it would be worse downstream.

"There’s a Cambodian saying that where there is water, there is fish. But there is technically no water left in this delta," the 45-year-old says.

The Mekong Delta was only formed 6,000 years ago, thanks to alluvial accumulation from the Mekong River, which runs more than 5,000 km from China.

The delta, home to the last 250 km of the river, became the third largest river basin in the world, though the youngest and far younger than the two larger basins, Bengal in India and Mississippi in the U.S., which have a history of hundreds of millions of years.

The Mekong River spans more than 5,000 km from China, including 250 km in Vietnam. Graphics by Hoang Khanh, Dang Le

The southern Vietnamese delta now spans over 40,000 square kilometers, and is home to 17.4 million people. It accounts for half of Vietnam’s rice output and 65% of its country’s aquaculture produce, and 17% of the GDP.

Farmers like Thom depend on the river for their livelihoods, but it has become more vulnerable than ever since the river’s wellbeing depends on impacts from upstream, and it is losing itself bit by bit for the past 20 years since the balance of alluvial addition and erosion was upset.

Before 1990 the river carried an average 160 million tons of fine silt and 30 million tons of sand and gravel to the delta, a crucial supply of resources strengthening the river, its estuaries and more than 30,000 km of channels.

But the supply has dried up in the last two decades, leaving the delta to suffer constantly from erosion.

For centuries the delta was expanding by 16 square kilometers, or nearly 3,000 football pitches, a year, but now it is losing five square kilometers annually.

Nguyen Huu Thien, who has spent more than 20 years studying the Delta, says it was formed since the alluvial accumulation and erosion happened in parallel, with the former being the dominant process.

According to studies, alluvium, sand and gravel from upstream flow 200 km downriver every year during the flood season from July to September. That means it takes 20-30 years for them to travel the whole 4,400 km from China to Vietnam. Most of the mixture serves to expand the delta while a small portion ends up in the estuaries and helps form an armor against erosion by sea waves.

However, there has been less accumulation since 1990, and erosion has become the dominant process since 2005, according to the Southern Institute of Water Resources Research.

In 1973-95 the delta was expanding by an average of 7.2 meters a year. This dropped to around 2.8 meters a year in the next decade. From 2005 to 2015 it was losing 1.4 meters a year, and now 68% of the delta’s coastline is suffering from erosion.

The length (in meters) of sediment addition and erosion along the Mekong Delta's coast. Graphics by VnExpress/Hoang Khanh

Half of Thom’s three-year-old house was swallowed by erosion in 2001.

Thien says the main reason for the constant erosion is the lack of alluvium, gravel and sand, the main materials that built up the delta.

According to the Mekong River Commission, alluvium flowing down the Mekong declined from 160 million tons in 1992 to 47.4 million tons in 2020.

It forecast the volume to drop to 4.5 million tons in 2040.

The main reason is the construction of hydropower dams on the river, starting with the Manwan Dam China built in 1995.

The Mekong River is set to have more than 400 hydropower dams, or one every 10 km, and it is predicted that they are going to hold back all the alluvium, sand and gravel, leaving the Vietnamese delta high and dry.

The number of erosion spots in the delta has increased from less than 100 a decade ago to around 600 now.

Sediment flow to the Mekong Delta reduces as new hydropower dams appear upstream. Videographics by VnExpress/Hoang Khanh

"The Mekong Delta is bleeding land much faster than other river deltas in the world," Marc Goichot, Lead of the Freshwater program at the WWF Asia Pacific, says

He says the Rhine River Delta in the Netherlands and the Mississippi River Delta in the U.S. encountered similar problems, but it took more than 100 years to turn so severe there as it took the Mekong Delta 20 years.

The Vietnamese delta is not only shrinking, it is also sinking, by more than 1 cm a year, three to eight times faster than the sea level rise.

A study by Dr Rafael Schmitt of the U.S. Stanford University and his colleagues published in 2021 suggested that 23-90% of the delta will be under water by 2100. The rate will depend on the volume of sediments, groundwater available and rise in sea level.

It means that in 80 years the area is likely to go back to how it was 6,000 years ago, gone and replaced by the sea.

Goichot says: "When deltas were formed, civilizations have appeared. It is scary to think about what would happen when river deltas disappear."

Thu Hang, Ngoc Tai, Hoang Nam

*This story is the first part of a four-part series. The second part "Take from the river, the river takes back" will be published Sept. 6, 2023.