The catchment area of Oued Larbaâ is located in the eastern prerif at the very eastern end of the large Innaouene basin, a stream of Oued Sebou (Lahrach. A et al, 1994)[1]. The objective of the study is to establish a quality comparison of the watershed during the dry and wet seasons. Eight water and sediment samples were taken during the two seasons, all along the main channel of Larbaa river, taking into account anthropogenic activities and the areas of confluence with its tributaries: Tarmasst, Taza, Larouireg, Defali and Jaouna. Physicochemical parameters in summer studied (pH, EC, TDS, nitrates, sulphates, chlorides), as well as metal contamination (Al, Zn, Ni, Pb, Ag, Fe, Cd, Cr, Cu, Mg, Mn). The results revealed on alkaline pH higher than 7, a significant electrical conductivity in several stations, but still conform to Moroccan standards. Laboratory analyses reflect that the waters of the basin have generally a good quality, except for a few peaks that indicate metallic contamination in Fe and Al. In order to better assess whether there is an anthropogenic effect influencing these results, and to remove the relationships between the different parameters studied, a statistical analysis with the PCA tool was carried out. This analysis made it possible to indicate Cr, Ni and Al as indices of pollution in certain areas. Although the results obtained are tolerant until now, but it is necessary to put in place an action plan to control the areas at risk of contamination during the years.