A former coal and iron valley river — 1890s reports found Tredegar discharging raw sewage directly into it — that has genuinely recovered a healthy wild brown trout population, confirmed independently by two clubs and a 2008 ecological survey (otters, breeding dippers, and bullhead, a clean-water indicator species, are all present). The honest caveat: current storm-overflow data shows the river still takes frequent, sustained sewage discharges around Tredegar and Blackwood-Pontllanfraith — real recovery, but not a finished job. No grayling here — the Grayling Society's own regional guide, thorough about this corner of Wales, doesn't list the Sirhowy.

• Carboniferous coal measures