The crack, in Binh My Commune in An Giang Province’s Chau Phu District, was discovered last Saturday when it was 30 meters long and a centimeter wide.

A preliminary investigation indicated it had been caused by a change in the flow of the Hau River, which runs just next to it, and weak ground.

It lengthened to 85m before it sheared off and collapsed into the Hau.

The province Chairman, Nguyen Thanh Binh, said local authorities had evacuated dozens of families and rerouted traffic, so there were no casualties.

The 142-kilometer National Highway 91 runs from Can Tho City to the Cambodian border in An Giang.

The delta, Vietnam's food basket, faces the threat of erosion at 562 riverine and coastal points involving a total length of 786 kilometers.

It is losing 500 hectares of land to erosion every year, and this is expected to directly impact the lives of one million people by 2050, according to the Ministry of Agriculture and Rural Development.

Experts have warned the erosion is likely to worsen.

Fingers have been pointed at overexploitation of groundwater and sand mining as the cause.

The Ministry of Construction last year drew up a proposal to build concrete barriers to safeguard 44,800 families living in erosion-prone areas in the region.