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