Llyn y Dywarchen is a ten-and-a-half hectare lake situated about two and a half miles west of mount Snowdon and four miles north west of Beddgelert. 'It's known as Llyn y Dywarchen (meaning Turf) after a little island in its waters. Medieval clergyman and chronicler Giraldus Cambrensis referred to the floating island on his journey through Wales in 1188.' In early 2013, 'most of its water was emptied to allow workmen to repair its walls.' The reservoir's dam is situated north of Clogwyngarreg, and is a 'stone-faced earth and stone constructed dam. It measures approximately 60-70m long by 5m wide at the top and tapers to 12m wide at the base' (NPRN 287158).
Additional source: Daily Post article entitled 'Plug to be pulled and water emptied from Snowdonia lake' published on 30th January 2013.
RCAHMW, 1st October 2018.