Off the central-southern Chilean coast (35°-38°S), the Gulf of Arauco is one of Chile's largest semi-enclosed coastal areas. It hosts industrialized activities within a highly productive zone of the Southern Humboldt Current System. One of the principal hydrodynamical forcings of the region is the Biobio River, whose discharge significantly influences coastal dynamics in the Gulf. The present work aims to study the impact of the Biobio River freshwater discharge on the circulation patterns in the Gulf of Arauco, using a high-resolution interannual simulation of the period 2013-2018. The simulation includes monthly interannual discharge from the significant four rivers for the study zone (Mataquito, Maule, Biobío, and Itata). The focus is primarily on wintertime (June-September), the highest freshwater discharges period. The modeled temperature and salinity fields were consistent with in-situ observations, presenting a moderate bias. The water masses highlighted in the TS diagrams, the temperature time series, and especially the currents near the Biobio River mouth were well represented in the simulation. It was found that the Biobio River strongly impacted the circulation in the Gulf of Arauco, intensifying the currents and causing a notable salinity decrease. Offshore zonal currents were intensified west of the Biobio River mouth, whereas southward alongshore currents were enhanced, especially during August. The influence of the Biobio River in the Gulf of Arauco was closely related to discharge strength. A strong relation between predominant southward downwelling-favorable wind stress and meridional currents was found, probably due to the formation of a buoyant coastal current strengthened by the wind-driven current during winter. Finally, surface buoyant waters associated with the river discharge generated a strong baroclinic zonal pressure gradient equilibrating the sheared meridional flow and enhancing the meridional ageostrophic pressure gradient, Reynolds stress, and near-surface vertical mixing of momentum.