River Vyrnwy

The River Vyrnwy is a river which flows through northern Powys, Wales, and Shropshire, England. The name derives from Severn, the river of which it is a tributary.
The river used to be sourced from the many rivers and streams running off the mountains surrounding the Vyrnwy valley. However, since the Lake Vyrnwy dam was built in the 1880s, the river has flowed directly from the base of the dam. The river runs for 39.7 miles , and the last 8 miles form part of the Welsh/English border between Powys and Shropshire. It eventually joins the River Severn near the village of Melverley.

Wikipedia