Statistical models are developed for the estimation of phosphorus concentrations for different seasons and locations using routinely monitored water quality parameters in the Grand River Basin. Two methods of modelling, namely the best subset model and the stepwise regression method based on R2 and F values, are described. The best subset modelling procedure enables comparison between full model (containing all the independent variables) and subset models (containing subsets of independent variables). For correlated independent variables, the best subset modelling procedure is shown to provide a better model than the stepwise regression procedure. The statistical modelling results indicate that suspended solids play an important role in the prediction of phosphorus levels and consequently decreasing suspended solids would decrease the growth of aquatic plants in the Grand River Basin. Key words: regression modelling, water quality, phosphorus, suspended solids, rivers, statistics.