- Location: Rhinog Mountains, north Wales
- Type: Lake
- Catchment area: 119 ha
- Annual rainfall: c. 2200 mm
- Lake area: -
- Lake altitude: 287 m
Situated at the southern edge of the Rhinog Mountains in north Wales, the catchment of Llyn Cwm Mynach covers an area of 119 ha and reaches a maximum altitude of 680 m. The solid geology of the catchment consists of Cambrian siltstones to the north and grits and greywackes of the Cambrian Rhinog formation to the south. The north and east of the catchment is dominated by amorphous blanket peats, whereas acid ranker soils characterise the south and west. There are areas of exposed rock on the steeper slopes to the west.
The lake and its catchment receive an annual rainfall of c. 2200 mm.
Approximately half of the catchment is forested with Japanese larch, Lodgepole pine and Sitka spruce planted between 1967 and 1973. The remainder is acid moorland characterised by Calluna and Vaccinium and utilised as rough grazing for sheep. The remains of short-lived small manganese mines are to be found the south-west of the catchment but there is no evidence to suggest these abandoned excavations have had any effect on water chemistry during monitoring. Both the lake and the catchment lie within the Snowdonia National Park.
Llyn Cwm Mynach lies at 287 m altitude. The lake consists of two distinct basins; the southern section comprises a shallow limb separated from the deeper northern section by an old and broken stone causeway. The maximum depth of 11 m occurs in the northern basin in a localised hollow. Discrete drainage to the lake is by three small inflows and it is drained to the south-east by a small stream which flows to the Mawddach estuary. At some stage in the past a wooden weir (now derelict) was constructed at the outflow, presumably to raise the level of the southern section of the lake.
Data Distribution and Associated Networks
The llyn has been designated as a site in the Environment Agency's Water Framework Directive Lake Surveillance Network. The UK UWMN contributes the biological and chemical data required for this statutory monitoring.