The Alyn rises on Llandegla Moors and crosses a carboniferous-limestone gorge at Loggerheads that genuinely swallows it — between Loggerheads and Rhydymwyn the river can run dry into natural swallow holes in low rainfall, worsened by the Milwr Tunnel, a Victorian lead-mine drainage tunnel that still permanently diverts around 23 million gallons a day out of the catchment to the Dee Estuary. Above and below that dry section it's real wild brown trout and grayling water — alkaline, gin-clear over gravel, and notably flashy after rain. Salmon and sea trout are occasionally seen at Rossett weir but don't hang around; this isn't a migratory-fish river.
- Limestone karst
