The Alwen Reservoir or Cronfa Alwen is a 5km long reservoir near Pentre-Llyn-Cymmer in Conwy County Borough, Wales. The bulk of the reservoir lies in Cerrigydrudion community, while the northern arm forms the boundary between Pentrefoelas, to the west, and Llansannan and Nantglyn, to the north and east. It is held back by the 27 metre high Alwen Dam, which impounds the Afon Alwen, and is 8km downstream from Llyn Alwen. It was built between 1909 and 1921, originally to supply water to the town of Birkenhead, near Liverpool in England.