Ranco Lake is located in Ranco Province of Chile. It is the largest lake of Los Ríos Region and the fourth largest in Chile. Administratively Ranco Lake is split into three communes; La Unión, Futrono and Lago Ranco. The lake host a series of islands of which Guapi Island is the largest. The western half of the lake is roughly circular while the eastern part has deep embayments. Geographically the lake lies in the Andean precordillera, the zone between the Chilean Central Valley and the Andes.
During the Miocene the lake depression was connected to the Pacific Ocean as an embayment of it. During this period a series of marine fossil bearing sediments known as Estratos de Lago Ranco were deposited in the lake area. During the quaternary glaciations the lake depression was covered by a large glacier lobe of the Patagonian Ice Sheet which left semicircular moraines around the lakes western shore.