They found a place to hide, stopped there, turned off all the lights, and waited in silence.
Then they suddenly started their boat to chase after three wooden boats and two steel vessels carrying sand, which was later measured at more than 120 cubic meters.
The sand thieves loudly urged each other to jump into the river and disappeared under cover of darkness, leaving just one man aged around 51 years.
Such hunts for sand thieves are one of several measures taken in recent years by authorities in the Mekong Delta to try and save what is left of heavily exploited rivers.
Sand theft has been rampant in the delta to serve the huge and growing construction industry.
According to the Ministry of Construction, the annual demand is for around 130 million cubic meters of sand while the authorized supply is only 62 million cubic meters.
There is no data for the volume of sand mined illegally except from individual cases that are busted.
In one such case, An Giang Province deputy chairman Tran Anh Thu was arrested last month for allegedly taking bribes of VND1.2 billion (US$50,000) to allow a business to mine 3.2 million cubic meters of sand above its licensed 1.5 million cubic meters.
To safeguard the country's limited supply, in 2009 the government for the first time banned construction sand and only allowed businesses to export sand taken from estuaries.
In 2017 it imposed a total ban on exports of all types of sand.
But all this could be too late.
Marc Goichot, lead of the freshwater program at the WWF Asia Pacific, said erosions are the result of destabilization of the delta.
"They are the more visible part, they are the tip of the iceberg."
"Prior to interventions 20-30 years ago, there was a balance. This was compensated by sediment coming from the river. Now you have a reduction in sediment.
"You can see the upstream deposition like credit to your account. And then people take sand out for construction, and that is the debit in your account. And now we know that the stock is limited to less than 10 years. So if nothing is done to increase the credit, or reduce the debit, then there will be a crash."
Dr Nguyen Nghia Hung, deputy head of the Southern Institute of Water Resources Research, said one reason for the delta's sand crisis is that there has never been a calculation of its actual supply, a task that is beyond the technical and financial prowess of local provinces.
The World Wildlife Fund Vietnam is trying to address this with its Sustainable Sand Mining Project (2021-2024), which aims to establish "a delta-wide sand budget to create a better understanding of the scope and impact of unsustainable extraction rates."
Ha Huy Anh, national manager of the project, said it "will prevent the Mekong Delta’s balance of sand from getting further in the red, and help us pay some debt to the river."
He said the project is expected to reduce riverbank and coastal erosion, salinity intrusion and high tides.
Futile efforts
To protect the Mekong Delta from erosion and other kinds of environmental degradation, the government has spent nearly VND11.5 trillion (US$479 million) since 2016 building anti-erosion works that stretch 246 km long.
Another VND4.77 trillion is going to be used to build more riverbank and coastal embankments in the near future.
But these efforts have not stopped erosion.
The incidence of erosion in the first seven months of this year was equal to that of last year.
A three-kilometer embankment along the Tien River, one of the two tributaries of the Mekong in Vietnam, has suffered four instances of erosion since it built three years ago, and only half of it remains intact now.
Duong Van Ni, a lecturer in the environment and natural resources department at Can Tho University, described it as typical evidence of "ineffective" embankment construction in the delta.
Building embankments along riverbanks and coasts is a waste of money and is like "building castles from sand" because they themselves might soon be swallowed by the river, he said.
Nguyen Huu Thien, an independent Mekong Delta researcher, said embankments are costly and do not always work, especially in places where the water is deep.
"We will never have enough money to chase after erosions."
Embankments should only be built when they are urgently needed to protect a residential area, he said.
Goichot said "the most critical and sustainable way" to build up the Mekong Delta’s resilience is to value sand and ensure there is enough sand to keep a balance in the river.
"That's how nature works for humans."
While resetting the balance in the Mekong River might take time, experts are urging authorities to evacuate communities in vulnerable areas. But that also requires money.
According to the Vietnam Disaster and Dyke Management Authority, around 20,000 families along the Mekong’s branches in Can Tho City and Dong Thap, An Giang, Vinh Long, and Ca Mau Provinces need to be urgently relocated and it would cost of tens of trillions of dong.
Ni pointed out that money is just one issue.
"The delta does not lack land for these people to settle down. So why have the authorities let them build houses close to riverbanks, only to scream about erosion and people losing houses every year?"
The fact that people continue to build houses along riverbanks shows that authorities have not taken erosion seriously or communicated to the public the risks, he said.
"People think that land close to the rivers is free, while officials are lackadaisical."
He said authorities should prohibit people from building along the banks of rivers, canals, etc. and move them all to safer areas.
This was recommended by scientists a decade ago, and it would save the provinces the need to spend on embankments, he added.
Ngoc Tai, Hoang Nam, Thu Hang
*This story is the final part of a four-part series. Read the previous three parts in the links below.
