A short alpine Esla tributary joining the parent river at Boca de Huérgano. The Yuso drains limestone country south of Riaño and gives the upper Esla system one of its best-known wild brown trout reaches. Pool-and-riffle structure on the middle stretches around Besande; pocket water higher up. Best fished from late May through July, after the snowmelt freshet has dropped back.
- Limestone