Thirlmere is a reservoir in the Borough of Allerdale in Cumbria and the English Lake District. 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.It runs roughly south to north and is bordered on the eastern side for much of its length by the A591 road and on the western side by a minor road. It occupies the site of a former natural lake: this had a fordable waist so narrow that it was (and is) sometimes regarded as two lakes. In the 19th century Manchester Corporation constructed a dam at the northern end, raising the water level, flooding the valley bottom, and creating a reservoir to provide the growing industrial city of Manchester with