UK Upland Waters Monitoring Network (UK UWMN) site. 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. Some data are managed by UK ECN Data Centre. Note: The area is the size of the catchment.