The aim of this paper deals with the study of the surface waters quality in the northwestern "Sistema Central" Range (Spain), in two different periods, spring and summer. The area is characterized by the existence of natural environments of great interest (western area of the Regional Park of "Sierra de Credos", Natural Parks of "Las Batuecas" and "Candelario" and the Protected Landscape of "El Rebollar"). The quality of the surface waters has been determined using the simplified index of water quality (ISQA). An hydrological mapping has been carried out having into account the ISQA values for these two seasons. The observed seasonal variability is mainly due to the variation of flows and the concentration of the chemical species in the water