The mean mechanical and chemical erosion rates of the Girou basin are calculated from the amount of suspended and dissolved material transported by the river. The chemical and mineralogical characteristics of the river load and its behaviour are studied during the water year 1980–1981. Finally, the origins of major compounds in the drainage basin are determined and the contribution of mechanical and chemical erosion to the fluvial transport of each compound are calculated. The separation of the streamflow components on the storm hydrograph allows the calculation of the suspended sediment concentration in the surface runoff.