Nguyen Van Thom is proud to say he is sensitive to all the changes in the Mekong and regrets to see that the river “lost its rhythm” for 20 years.
Thom’s family lived for years near Tonlé Sap Lake in Cambodia before moving downstream to settle in An Giang Province in southern Vietnam.
When his Cambodian neighbors complained about declining fish stocks in 2019 and the fact that the water in the lake was no longer brownish but rather blue, he knew it was bad news since it meant there were had little silt in the water, and it would be worse. downstream.
“There’s a Cambodian saying that where there’s water, there’s fish. But technically, there’s no water left in this delta,” the 45-year-old explained.
The Mekong Delta was formed only 6,000 years ago, thanks to the alluvial accumulation of the Mekong River, which flows more than 5,000 km from China.
The delta, home to the last 250 kilometers of the river, has become the third largest river basin in the world, although the youngest and much younger than the two largest basins, the Bengal in India and the Mississippi in the United States. , which have a history of several hundred years. millions of years.
Vietnam’s southern delta now spans more than 40,000 square kilometers and is home to 17.4 million people. It accounts for half of Vietnam’s rice production, 65% of the country’s aquaculture production and 17% of GDP.
Farmers like Thom depend on the river for their livelihoods, but it has become more vulnerable than ever as the well-being of the river depends on impacts from upstream, and it has been slowly being lost for 20 years since the balance between alluvial input and erosion. was angry.
Prior to 1990, the river carried an average of 160 million tonnes of fine silt and 30 million tonnes of sand and gravel to the delta, a crucial source of resources bolstering the river, its estuaries and more than 30,000 km of channels.
But the supply has dried up over the past two decades, leaving the delta to constantly suffer from erosion.
For centuries, the delta has grown by 16 square kilometers, or almost 3,000 football fields, per year, but it is now losing five square kilometers per year.
Nguyen Huu Thien, who has spent more than 20 years studying the Delta, says it formed because alluvial accumulation and erosion occurred in parallel, with the former being the dominant process.
According to studies, alluvium, sand and gravel from upstream flow 200 km downstream each year during the flood season, from July to September. This means it takes them 20 to 30 years to travel the 4,400 km between China and Vietnam. Most of the mixture serves to widen the delta while a small portion ends up in estuaries and helps form armor against wave erosion.
However, accumulation has declined since 1990 and erosion has become the dominant process since 2005, according to the Southern Institute of Water Resources Research.
Between 1973 and 1995, the delta widened by an average of 7.2 meters per year. This figure fell to around 2.8 meters per year over the next decade. From 2005 to 2015, it lost 1.4 meters per year and today 68% of the delta’s coastline suffers from erosion.
Half of Thom’s three-year-old house was swallowed up by erosion in 2001.
According to Thien, the main reason for the constant erosion is the lack of alluvium, gravel and sand, the main materials that made up the delta.
According to the Mekong River Commission, alluvial deposits flowing along the Mekong have increased from 160 million tonnes in 1992 to 47.4 million tonnes in 2020.
It predicts the volume will drop to 4.5 million tonnes in 2040.
The main reason is the construction of hydroelectric dams on the river, starting with the Manwan Dam in China built in 1995.
The Mekong is expected to have more than 400 hydroelectric dams, or one every 10 km, and it is expected that they will retain all the alluvium, sand and gravel, leaving the Vietnamese delta high and dry.
The number of erosion points in the delta has increased from fewer than 100 a decade ago to around 600 today.
“The Mekong Delta is bleeding land much faster than other river deltas in the world,” says Marc Goichot, freshwater program manager at WWF Asia-Pacific.
He claims that the Rhine Delta in the Netherlands and the Mississippi Delta in the United States had similar problems, but it took more than 100 years for them to become as bad as it took 20 years for the delta of the Mekong.
The Vietnamese delta is not only shrinking, it is also sinking more than a centimeter a year, three to eight times faster than sea level rise.
A study by Dr Rafael Schmitt from the American University of Stanford The university and colleagues published a paper published in 2021, suggesting that 23 to 90 percent of the delta will be underwater by 2100. This rate will depend on the volume of sediment, available groundwater and elevation of the sea level.
This means that in 80 years the region will likely return to what it was 6,000 years ago, gone and replaced by the sea.
Goichot says: “When deltas formed, civilizations appeared. It is scary to think what would happen when river deltas disappear. »
Game Hang, Ngoc Tai, Hoang Nam
*This story is part one of a four-part series. The second part “Take the river, the river picks up again“will be released on September 6, 2023.