Río Diguillín is a tributary of the larger Itata basin, flowing through Chile's Biobío region between approximately 35° and 37°S latitude. As one of the sub-basin rivers feeding into the Itata watershed, the Diguillín drains terrain that rises toward the Nevados del Chillán volcano, which reaches an elevation of 3,213 meters above sea level — the highest point in the entire Itata basin. This volcanic and Andean geography shapes the river's character, producing a high-gradient stream system adapted to fluctuating flows. The watershed as a whole spans roughly 11,200 km² and supports a remarkable assemblage of native freshwater fish, many of which are found nowhere else on Earth. The Diguillín's waters are part of a biologically significant system where Chile's extraordinary level of freshwater fish endemism — approximately 80% of species are endemic to the country — makes every encounter with native fish a genuinely rare experience.