The Riosa arrives into the Nalón system bringing steep Cantabrian descent and mixed lithology character. The upper reaches are step-pool slate and quartzite; the descent is steep and response to rainfall pronounced. The middle Riosa allows pool-riffle sequences to develop on cobble; these pools are known and respected. The river maintains its responsive character — quick to rise, quick to fall, connected fully to mountain weather — throughout.

Wading: Wet boulder tops and lichen glazed bedrock steps

In the mountain-to-sea trout rivers of Asturias, olives and other mayflies do much of the work in spring whenever levels and colour allow, followed by strong evening caddis and steady buzzer life through early and mid-summer, with terrestrials and mixed surface food from wooded, pastoral banks rounding out the diet whenever flows drop back into safe wading range.