Transverse mixing under steady conditions is studied by introducing the mass streamline as the streamline of the mass density vector field. The two equations derived for it allow calculations of the concentration field, or the dispersion coefficient under different assumptions if the concentrations are given. The correctness of numerical methods developed was tested. Tracer studies have been performed on five different streams (Q = 0.25 - 2000 m3s-1) and data published have also been used. The methods based on the concept of mass streamline were applied, and the results were compared to observed concentrations. The limitations of the two-dimensional treatment have been examined finally. Key words: streams, channels, hydraulics, diffusion, dispersion, turbulence, water, water pollution, numerical methods.