Thirlmere is a reservoir in the Borough of Allerdale in Cumbria and the English Lake District. It runs roughly south to north, with a dam at the northern end, and is bordered on the eastern side by the A591 road and on the western side by a minor road.
Thirlmere was constructed in the 19th century by the Manchester Corporation to provide the burgeoning industrial city of Manchester with water supplies. The 96 mile-long Thirlmere Aqueduct still provides water to the Manchester area.
Before the construction of the reservoir the glacial valley 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 in the valley was raised by construction of a dam by Manchester Corporation Waterworks at the northern end of Thirlmere, in 1890–1894. The reservoir supplied water to Manchester via the Thirlmere Aqueduct which is 95.9-mile (154.3 km). John Frederick Bateman was advisor to the corporation for both projects.