Thirlmere, lake, c.3 mi (5 km) long, in the Lake District, Cumbria, NW England, near Keswick. In 1894, through the construction of a high dam (which raised the water level c.50 ft/15 m) and an aqueduct c.100 mi (160 km) long, Thirlmere became a source of water supply for Manchester.

Thirlmere is a reservoir in the Lake District National Park, Cumbria, England. It runs roughly south to north and is bordered on the eastern side by the A591 road and on the western side by a minor road.


Prior to the construction of the reservoir the site was occupied by two smaller lakes - Leathes Water and Wythburn Water. The growth of the industrial city of Manchester during the 19th century had led to an increased demand for water. The water-level was raised by construction of a dam by the Manchester Corporation at the northern end of Thirlmere, in 1890–1894. The reservoir was then able to supply water to Manchester via the Thirlmere Aqueduct, roughly 100 miles long. John Frederick Bateman acted as advisor to the corporation for both projects.

There was strong local opposition to the construction of the lake and the Thirlmere Defence Association (TDA) was formed to oppose the parliamentary act which was required before workcould begin. The TDA opposed on the basis that raising the water level by 50 feet would submerge the dramatic cliffs which then surrounded the lake and a receding shoreline in summer would expose the smelly and unsightly lake bed. The organisation managed to stall the reading of the act in parliament in 1878 but the act was passed at the second reading in 1879..


The name is sometimes also applied to the whole valley, which connects Grasmere in the south with the Vale of Keswick in the north. The highest point in the valley is Dunmail Raise. The A591 runs the length of the valley and goes over Dunmail Raise.

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.


The reservoir and surrounding forested valley is owned and managed by United Utilities, a private water and utility company.


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