Abstarct: Rivers and surface water are two major media to release pollutants into the environment and consequently to the human food chain. Rivers receive wastewaters from human activities, and due to advection - diffusion are capable of dilute the pollutants and reduce the concentration of contaminants. This property, known as self attenuation, enables the release of the contaminated wastewaters into the rivers while the average water quality in the river remains acceptable for various purposes. The dilution process takes a minimum distance which depends on the contaminant concentration and entrance position as well as, hydraulic conditions such as velocity and angle of attacks. Reduction of this length is of engineering interest since the region of high concentration pollution is confined. The most preferred method to enhance the mixing process is elevation of turbulence in the flow. One of the natural techniques to do so is sudden expansion which accelerates the diffusion process. In this study, the effect of different entry points of pollutant , expansion ratios and flow rates was studied through a two dimensional numerical model in which the distribution of pollution in an expansion was programmed in FORTRAN and MATLAB. The results showed that a sudden expansion with a ratio in the range of 0.1 to 0.2 is more efficient.