Vietnam's Mekong Delta may run out of sand by 2035, a leading expert warned Thursday, as sand-mining and dams strip the "rice bowl" region, putting people, crops and the economy at risk.

The delta, a biodiversity hotspot where the Mekong empties into the South China Sea, provides food and livelihoods for tens of millions of people.

Hydropower dams further up the Mekong are reducing the flow of sand downstream, playing a major role in the rapid decline of sand under the riverbed that is critical to the delta's resilience.

Mining sand to feed Vietnam's booming construction sector is fast depleting resources, according to a major WWF report on the problem.

At the current extraction rate of 35-55 million cubic metres a year, "the exploitable stock under the riverbed... can only last as far as a decade", the "Sand Budget for Viet Nam Mekong Delta" report warned.

Report co-author Sepehr Eslami told AFP that if the sand runs out, there would be a 10 percent increase in areas affected by salt intrusion -- a phenomenon that has already caused huge damage to rice crops in the freshwater Mekong.

Eslami said sand depletion would also lead to "more river bank erosion and larger tides that lead to more flooding and erosion".

In the context of climate change, and the threat of rising sea levels, the shortage of sand represents an existential threat to the delta, the WWF warns.

The Mekong Delta is one of the world's most rapidly transforming and lowest-lying deltas, with a reserve of up to 550 million cubic metres of exploitable sand.

The study said only four million cubic metres of sand flowed down to the delta last year -- well down on the average of up to seven million.

The amount of sand being extracted each year is likely to be even higher than the study reported because the research did not calculate the volume mined at night, when many illegal dredgers are known to operate.

In August, police arrested a senior official in An Giang, a Mekong Delta province, for allegedly taking bribes of $50,000 from a mining firm.

The Trung Hau 68 Company mined more than 4.7 million cubic metres of sand despite having a licence to extract just a third of that, police said.

People's homes and businesses across the Mekong Delta have disappeared into the sea due to coastal erosion, in part caused by disappearing sand.

The ministry of agriculture and rural development says nearly 800 erosion spots have been reported, stretching across a total of 1,134 kilometres of sea and river banks in the Mekong Delta, since 2016.