Abstract: The most important parameter is usually the degree of dilution in river multiport diffusers discharge. Numerical simulation and laboratory experiments are used to predict the flow characteristics of river diffusers. Continuity equation, momentum equation and Bernoulli's equation are applied to develop the calculation formulas for the degree of nearfield dilution in the recirculated flow, the non-recirculated flowand the stratified flow. The results of calculation are in good agreement with laboratory experiments.