Haweswater is a reservoir in the English Lake District built in the valley of Mardale in the county of Cumbria. The controversial construction of the Haweswater dam was started in 1929 after Parliament passed an Act giving Manchester Corporation permission to build the reservoir to supply water for the urban conurbations of north-west England. At the time there was much public outcry about the decision as the valley of Mardale was populated by the farming villages of Measand and Mardale Green and the construction of the reservoir would mean that these villages would be flooded and lost and the population would have to be moved. In addition the valley was considered one of the most picturesque in Westmorland and many people thought it should be left alone.

Originally Mardale contained two small natural lakes, High Water and Low Water, but the building of the dam raised the water level by 29 metres (95 feet) and created a single lake six kilometres (four miles) long and around 600 metres (almost half a mile) wide. The dam wall measures 470 metres long and 27.5 metres high, and at the time of construction it was considered to be cutting-edge technology as it was the first hollow buttress dam in the World, being constructed using 44 separate buttressed units joined by flexible joints. There is a parapet 1.4 metres (56 inches) wide running the length of the dam and from this, tunnelled supplies can be seen entering the reservoir from the adjoining valleys of Heltondale and Swindale. When the reservoir is full it holds 84 billion litres (18.6 billion gallons) of water.

Prior to the valley being flooded in 1935 all the farms and dwellings of the villages of Mardale Green and Measand were demolished as well as the centuries-old Dun Bull Hotel at Mardale Green. The village church was dismantled and the stone used in constructing the dam; all the bodies in the church yard were exhumed and re-buried at Shap. Today when the water in the reservoir is low, the remains of the submerged village of Mardale Green can still be seen as stone walls and the village bridge become visible as the water level drops.

Manchester Corporation built a new road along the eastern side of the lake to replace the flooded highway lower in the valley, and the Haweswater Hotel was constucted midway down the length of the reservoir as a replacement for the Dun Bull. The road continues to the western end of Haweswater where a small car park his been built; this is a popular starting point for walkers who want to climb the surrounding fells of Harter Fell, Branstree and High Street.

The Haweswater valley is the only place in England where Golden eagles nest: there is a RSPB (Royal Society for the Protection of Birds) observation post in the remote valley of Riggindale where the pair have their eyrie. A pair of eagles first nested in the valley in 1969 and the male and female of the pairing have changed several times over the years, sixteen chicks have been produced. The female bird disappeared in April 2004 leaving the male on its own. However, the RSPB are hoping a replacement female will be drawn to the area.

Famous Lake District writer and fell walker Alfred Wainwright had these words to say on the construction of the Haweswater dam in his Pictorial Guide to the Far Eastern Lakeland Fells:

“If we can accept as absolutely necessary the conversion of Haweswater [to a reservoir], then it must be conceded that Manchester have done the job as unobtrusively as possible. Mardale is still a noble valley. But man works with such clumsy hands! Gone for ever are the quiet wooded bays and shingly shores that nature had fashioned so sweetly in the Haweswater of old; how aggressively ugly is the tidemark of the new Haweswater!”.