In the south-central zone of Chile, the Renegado River presents irregular behavior during the dry season, with dry and high-flow sections along its course at the same time. The objective of this study was to identify interactions between surface and groundwater that explain this behavior. Through a flow measurement campaign along the river, an analysis of the geological conditions of the watershed and the use of a numerical groundwater flow model, the interactions between the water table and the river were described and represented. As a result, it was identified that the combination of topography and geology given by the volcanic nature of the catchment leads the river to gain or lose surface water along its course. It is recommended that the interaction between surface and groundwater be analyzed to address hydrological design studies and water management in volcanic systems with similar characteristics