The River Banwy is a river about 19 miles long in Powys, Wales. It is a tributary of the River Vyrnwy. The Banwy rises in the hills near the pass which takes the A458 road between Mallwyd and Welshpool. The river is called Nant Cerrig-y-groes at its source near Moel y Llyn. Then flowing east, it joins a number of lesser streams before reaching Pont Twrch …The River Banwy is a river about 19 miles long in Powys, Wales. It is a tributary of the River Vyrnwy. The Banwy rises in the hills near the pass which takes the A458 road between Mallwyd and Welshpool. The river is called Nant Cerrig-y-groes at its source near Moel y Llyn. Then flowing east, it joins a number of lesser streams before reaching Pont Twrch near the village of Y Foel, at its confluence with the river Twrch. Two miles further on, it is joined by the river Gam, which flows down from the Nant yr Eira, between Y Foel and Llangadfan.