Marchlyn Mawr Dam is a natural lake dammed to form a reservoir at an elevation of 643m (2,109ft) above the sea level, located in Gwynedd, in north-west Wales. It’s one of the highest mountain roads of the country.
The road to the summit, in Snowdonia, a mountainous region in north west Wales, is asphalted but pretty narrow. The road to the reservoir is owned by the power/water company and closed to cars. Starting from Tyddyn Dicwm, the ascent is 10.1 km long. Over this distance, the elevation gain is 546 meters. The average percentage is 5.4 %. The maximum gradient is 12.0%.