The Sirhowy River has its source on the slopes of Cefn Pyllau-duon above Tredegar. After flowing through Siôn-Sieffre's Reservoir it turns south through Tredegar and then Blackwood and Pontllanfraith. It turns eastwards near Cwmfelinfach and joins the River Ebbw near Crosskeys. From the early 19th century, the Sirhowy Valley was a highly industrialised area.
