The natural lake Marchlyn Mawr was dammed to form the upper reservoir of the Dinorwig Pumped Storage Scheme, the largest such hydroelectric scheme in Europe at the time of its construction. The dam is rock-filled and sealed with dense asphaltic concrete. Its crest level has been raised to increase storage capacity.
Llyn Marchlyn Mawr formerly held about 2.5 million cu m of water behind a natural moraine dam some 40m high. In the 19th century, the dam was enlarged a little and the lake used for public water supply.
The modern dam was constructed on the rock sill of the cwm (valley), on the north side of the lake, incorporating the moraine. It raised the water level by 33m to create a reservoir with a maximum capacity of 9.2 million cu m. The live storage of 6.7 million cu m is released to drive the hydroelectric turbines of Dinorwig's underground power station