Thirlmere is a reservoir located in the Lake District National Park, Cumbria, England. Running roughly south to north it is bordered on the eastern side by the A591 road and on the western side by a minor road.
Before 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 nineteenth century had led to a greatly increased demand for water. The water-level was raised by construction of a dam by the Manchester Corporation at the northern end of Thirlmere, between 1890-1894. The reservoir was then able to supply water to Manchester via the Thirlmere Aqueduct, approximately 100 miles long. John Frederick Bateman was advisor to the corporation for both projects.
There was considerable local opposition to the construction of the lake and the Thirlmere Defence Association (TDA) was formed to oppose the parliamentary act which was needed before workcould begin. The TDA opposed on the basis that raising the water level by 50 feet would submerge the cliffs which then surrounded the lake and a receding shoreline in summer would expose the smelly and unsightly lake bed.
The organisation managed to delay the reading of the act in parliament in 1878 but eventually it was passed at the second reading in 1879.
The name is from time to time also applied to the whole valley, which connects Grasmere in the south with the Vale of Keswick to the north. The tallest point in the valley is Dunmail Raise. The A591 runs the full length of the valley and goes over Dunmail Raise.
To the east of Thirlmere lies the Helvellyn ridge. West of Thirlmere are a number of fells; for example, Armboth Fell and Raven Crag both of which give views of the lake.
The reservoir and the surrounding valley and its forests are owned and managed by United Utilities, a private water and utility company.
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