| Thirlmere | |
|---|---|
 Seen from Steel Fell at the southern end of the lake | |
  Thirlmere Location in the Lake District National Park | |
| Location | Lake District National Park, Cumbria, England |
| Coordinates | 54°32′N 3°04′W / 54.533°N 3.067°W |
| Lake type | Reservoir |
| Primary inflows | Launchy Gill, Dob Gill, Wyth Burn, Birkside Gill |
| Primary outflows | Thirlmere Aqueduct (engineered off-take); St John's Beck (natural) |
| Basin countries | England |
| Max. length | 6.05 km (3.76 mi) |
| Max. width | 0.7 km (0.43 mi) |
| Surface area | 3.25 km2 (1.25 sq mi) |
| Max. depth | 40 metres (131 ft) |
| Shore length1 | 15 km (9.3 mi) |
| Surface elevation | 178 m (584 ft) |
| Islands | 2 |
| 1 Shore length is not a well-defined measure. | |
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 water supplies via the 96 mile-long Thirlmere Aqueduct.
The reservoir and the aqueduct still provide water to the Manchester area, but under the Water Act 1973 ownership passed to the North West Water Authority; as a result of subsequent privatisation and amalgamation they (and the catchment area surrounding the reservoir) are now owned and managed by United Utilities, a private water and wastewater company.