Estero El Sauce is a small coastal estuary located in the Valparaíso Region of Chile, draining into the Pacific through a lowland mouth zone typical of the region's many minor watercourses. Like other esteros in this part of Chile, it flows through a landscape shaped by the coastal range and Mediterranean-climate valleys characteristic of central Chile. The estuary and its mouth zone represent an ecologically significant transition between freshwater and marine environments, supporting a mix of native, endemic, and introduced fish species. Notably, Estero El Sauce stands out among the region's watercourses for the dominance of introduced species over native ones at its mouth — a pattern observed across several esteros in the Valparaíso Region but particularly pronounced here.