Late last month they had discovered the groups in 13 boats mining the river, with Vuong Xuan Thanh, 51, Vu Tien Dung, 57, Vu Tien Manh, 51, and Vu Van Ngoc, 40, heading them.

Thanh had eight people working for him, including two boat owners, and it is not known yet how much they earned from the mining.

Dung’s group reportedly earned VND3.7 billion ($160,000) this year alone.

Manh’s group directly sold sand to boats in the river and the police estimated the value of sand in his possession at the time of the arrest at VND1.2 billion ($51,850).

The police do not know how much Ngoc and his group, who were found mining from his boat, have earned either.

There have been reports that illegal sand miners took advantage of the stay-at-home orders and the fact the public gaze was not on them as Vietnam ramped up its Covid-19 measures.

Local residents and authorities have been sounding the alarm about rampant mining in the Red River, which flows from Yunnan Province in southwestern China through northern Vietnam into the Gulf of Tonkin.

Upstream dams that hold back alluvium and overexploitation of sand are two reasons the riverbed has sunk, according to the Vietnam Academy for Water Resources.

Last month the government issued a regulation on sand and gravel management in rivers that tasks authorities at all levels with responsibility for curb illegal mining.