A group of five men in Mong An Commune, Bao Lam District, were getting sand for a construction project that afternoon when the flash flood arrived and caught them off guard.
Four of them were stuck in the stream while the other managed to escape.
A few minutes later a wall of water swept away the four.
Authorities in Bao Lam said one body was found late Tuesday night and rescue workers are still searching for the remaining three.
As many as 100 people, including civilians, have been mobilized for the search.
The National Center for Hydro-Meteorological Forecasting said between 7:00 p.m. Monday and Tuesday evening Cao Bang recorded rainfall of 80mm.
The center said the northern region, including Hanoi, would continue to receive heavy rains until Wednesday under the combined impact of low pressure and wind convergence areas.