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.

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