The River Rhymney rises on the southern edge of the Brecon Beacons before descending steeply through the town of New Tredegar towards Ystrad Mynach and then onwards across a flat plain before entering the Severn Estuary to the east of Cardiff.

It covers a distance of 30 miles. The catchment can be divided into two main parts: a steep-sided, wet, mountainous upper valley, and a flatter wider valley below Machen, where the river assumes a lowland meandering character.

While the river used to flow black with coal dust during the mining era, substantial improvements over the last 40 years have followed pit closures. The Rhymney now supports a healthy stock of grayling and natural brown trout.