|Location||Conwy County Borough, Wales|
|Max. length||5 km (3.1 mi)|
The Alwen Reservoir or Cronfa Alwen is a 5 km 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 8 km downstream from Llyn Alwen. It was built between 1909 and 1921, originally to supply water to the town of Birkenhead, near Liverpool in England. Today it is part of the River Dee regulation system and is operated by Welsh Water.
The dam is a gravity-arch masonry dam. The "first stone" of the dam records that the engineers were Sir Alex. Binnie, Son and [George] Deacon and the contractors were Sir Robert McAlpine. A large water treatment facility was built below the dam, and a cast iron underground aqueduct laid to Birkenhead. The original water treatment buildings are still standing, but the equipment inside has long gone, replaced by modern plant in a new building.
The operators, Welsh Water have a scheme to teach children about the importance of water, linked to the National Curriculum Key Stage 2 and offer an on-site classroom and guided tours of the water treatment works to schools. The nearby Outdoor Education Centre at Pentre-Llyn-Cymmer accommodates children during school trips.
The reservoir is very close to Llyn Brenig. It is at the end of a valley, and has no through routes around it for vehicles. However the reservoir has an extensive network of trails (public footpaths as well as forest roads suitable for horse riding, mountain biking and walking).
Footpaths around the reservoir were upgraded in 2005 and 2006, providing better access to the general public and allowing greater exploration of the North and South Alwen forests.
Fishing is permitted through permit (which can be purchased from the visitors centre at the nearby Llyn Brenig visitors centre).
Access is via the B4501, turning north off the A5 at Cerrig-y-drudion.