Embalse Ralco is a large hydroelectric reservoir located on the Biobío River in the Biobío Region of Chile, situated in the Andean foothills at high elevation. Created by the Ralco Dam, which became fully operational in 2005, the reservoir represents one of two major impoundments on the upper Biobío River — the other being the Pangue Dam, operational since 1997. Together, these structures have fundamentally transformed what was once a free-flowing Andean river into a managed hydropower system. The reservoir sits within a dramatic landscape of steep Andean terrain, and its waters reflect the surrounding mountain environment. The Ralco reservoir functions as an annual storage facility, meaning its water levels fluctuate seasonally as water is stored and released to meet hydropower generation demands, which significantly shapes the fishing experience throughout the year.