The Aller arrives into the Nalón from the northeast carrying coal-country character — sandstone, slate, and the legacy of mining in its catchment. The upper reaches are step-pool through mixed lithology; the descent is moderate and the spate response pronounced. The water sometimes carries the darkened signature of coal-stained fines. The middle Aller settles into pool-riffle on cobble. The river's character is distinct from pure slate or pure limestone systems — it carries the mark of its geology and recent industrial history. The wading demands care in upper sections; the pools offer more approachable fishing once the river opens slightly.

Wading: Slippery embedded cobble and algal coated slate 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.