Natural History
Situated in a limestone area, Malham Tarn itself mainly lies on a bed of silurian slate which is covered with marl deposits. The lake's basin was dammed by a moraine at the end of the last glacial period, approximately 10,000 years ago. It used to be about twice its current size, having shrunk due to silting at the western shore; this has formed a boggy region called Tarn Moss. Following deforestation during the Iron Age, the land surrounding the lake has been used for grazing which has prevented further tree growth. An embankment and sluice gate were added to the lake in 1791 by Lord Ribblesdale; this has had the effect of raising the level of the lake by approximately 1 metre (3.3 ft). The average annual rainfall over the catchment area is 1,542.5 millimetres (61 in).
The lake is home to six species of fish, as well as white-clawed crayfish, Great Crested Grebes, Moorhens, Coots, Tufted Ducks and Teal. A number of waders such as Redshanks, Curlews, Lapwings and Oystercatchers breed in the surrounding area. Two rare benthic copepods, Bryocamptus rhaeticus and Motatia mrazeki, are found in the lake, along with 22 species of molluscs—nine of which are found at their highest altitude in Britain. The lake also contains a number of submerged aquatic plants, while the surrounding area is home to a diverse number of plants including wild cranberry, bearberry, crowberry, dark-leaved willow and Purple Moor Grass.
The lake is located in the Malham and Arncliffe Site of Special Scientific Interest which was established in 1955. In 1992, the lake and its wetlands were designated as a National Nature Reserve. The lake was listed as a Ramsar Convention site in 1993. It is also in the Craven Limestone Complex Special Area of Conservation.
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