In the present paper the problem of the stream and intensive mass transfer phenomena in the initial section of a rectangular channel is investigated by numerical simulation. The mathematical model is based on the 2D Navier-Stokes equations and the convective-diffusion equation with the boundary conditions over the channel wall, which initiates the rising of the cross-flow due to the high concentration gradients near the wall. The non-linear hydrodynamical effects rising from the intensive mass transfer process are obtained. They are of the same order as the effects of the evolution of the flow along the channel. The results about the mass transfer kinetics are found.