Río Mulchén flows through the Biobío Region of Chile, draining into the broader Río Bureo watershed before ultimately contributing to the Río Biobío system. The river passes through the comuna of Mulchén, which accounts for the vast majority of its watershed area, with smaller portions touching the comunas of Negrete and Los Ángeles. The surrounding landscape is characteristic of south-central Chile's interior valleys and foothills, offering a rugged and scenic setting for anglers exploring this lesser-known waterway. The river's position within the Biobío Region places it in one of Chile's most storied fishing territories, a zone long associated with quality trout angling in freestone streams fed by Andean rainfall and snowmelt.