UK ECN and UK UWMN site. Llyn Llagi occupies a north-facing corrie in the central area of the Snowdonia region of North Wales. The lake lies at 380 m beneath a steep backwall and comprises a deep, almost circular basin (maximum depth 16.5 m) bordered by an extensive shallow (1 m deep) rim. The lake covers an area of 5.67ha and the primary inflow constitutes the outflow stream from Llyn yr Adar. The lake drains to the north-west to the Nanmor valley. The catchment (157 ha) consists primarily of Ordivician slates and shales of the Glanarfon series. The backwall is composed of a large doleritic intrusion with small intrusions of fine microgranites and volcanic tuff. The catchment soils are mainly stagnopodsols and gleys, interspersed with blanket peats. The vegetation is characterised by Calluna (heather), Molinia (purple moor-grass) and Eriophorum (cotton grass), and the catchment is grazed at a low intensity by sheep. The lake and much of the catchment lie within a designated Site of Special Scientific Interest (SSSI). Llyn Llagi is classified as having high acid deposition.