Thirlmere is a reservoir in the Borough of Allerdale. The Helvellyn ridge lies to the east of Thirlmere. To the west of Thirlmere are a number of fells; for instance, Armboth Fell and Raven Crag both of which give views of the lake and of Helvellyn beyond

Thirlmere, at 3.5 miles long, 1.2 mile wide and 158 feet deep, was originally two smaller lakes. In facts these were purchased by Manchester City Corporation Waterworks in 1889. The area was dammed with a dam whose greatest height is 104 feet, and the area became one vast reservoir. Over 2000 acres of spruce and larch were planted from 1908. Although these have lost some of their austere appearance, with more broad-leafed trees being planted now.

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