Maentwrog Dam was originally built as part of the Maentwrog Hydro Electric Scheme (NPRN: 33605), constructed by North Wales Power Company in the 1920's. Four dams were constructed, impounding the water for Llyn Trawsfynydd. The principal dam, crossing Afon Prysor at the northern end of Llyn Trawsfynydd, was in it's time the largest arch dam to be built in Britain. The dam was 96ft high and 37ft at the base. The dam was rebuilt in 1987.

Photographed during aerial reconnaissance by RCAHMW on 16th June 2010.
L. Osborne, 11th October 2011.