The Ser is a smaller slate-fed river in the eastern Asturian system, spate-responsive and amber-toned. The upper reaches are step-pool through Cantabrian slate; the descent is steep and response to rainfall immediate. The Ser maintains its mountain character throughout — responsive, slate-stained, belonging fully to the Cantabrian system. The pools available are brief; the character is descent and response.

Wading: Slick wet slate slabs at step lips

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.