A numerical method developed in the last two years is used to solve the diffusion equation with a delta-function as initial value. The two-dimensional case of a steady line source in a stream is considered. The velocity profile is logarithmic and the diffusivity is parabolic but any profile could be used. The source can be set anywhere from the bed up to the free surface. The results are given in a dimensionless form and a significant set of plots is automatically produced. The method could be easily applied to more complicated cases (unsteady, point source, etc.). (Author)