This paper proposes a new equation for predicting the transversal mixing coefficient giving the mean values of the longitudinal mixing coefficient, the quantity of flow, the depth of flow and the ratio width to depth of the river. This equation is developped from the relation between the transversal and the longitudinal coefficient proposed by the authors in 1994 taking into account all the data found in litterature of all the mixing coefficients (transversal and longitudinal) measured simultanously with the same hydraulic conditions. The proposed formula expresses the transversal coefficient as a fonction of the longitudinal coefficient. This new formula is more representative of the experimental results and confirms the trend of recent years to use the product of coefficients (KlKtrather than Kl/Kt) to describe the interaction between the transversal and the longitudinal mixing coefficients.