'The second largest manmade lake on Anglesey, Llyn Cefni is located fairly centrally on the island, roughly one mile to the north of the town of Llangefni. A dam was built on the Afon (river) Cefni in the 1940's, creating the reservoir to supply drinking water to the island. A currently disused railway called Lein Amlwch cuts the lake in two. This railway used to run from Amlwch Port to the village of Gaerwen where it joined the main line.
The Cefni Angling Association, established in 1952, manages the reservoir as a fishery, with part of the north eastern half being managed as a nature reserve. Two main rivers feed into Llyn Cefni, the Afon Frogwy on the west side and the Afon Erddreiniog entering on the north east side. This river links the lake to the Cors Erddreiniog National Nature Reserve, which is an important fenland area.'
Source: www.visitanglesey.co.uk, accessed on 17th July 2018.