The Caudal merges from significant tributaries to arrive into the Nalón system. The upper reaches show mixed lithology — slate, quartzite, sandstone — in step-pool confines. The descent is steep and response to rainfall pronounced. The middle Caudal opens into pool-riffle sequences on cobble. The river carries the blended character of multiple tributary systems, and that diversity shows in the water — sometimes amber with slate, sometimes clearer with limestone influence, depending on which tributary has most recently delivered spate flow. The wading demands attention throughout.

Wading: Slippery embedded cobble and anthropogenic debris

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.