Wastwater is a lake in theLake District National Park. The lake is approximately  3 miles long and 600 metres wide, and is located in theWasdale valley. It is the deepest lake in England, at 79 metres), and is owned by theNational Trust.

It is one of the finest examples of a glacially 'over-deepened' valley. The surface of the lake is about 200 feet above sea level, while its bottom is over 50 feet below sea level.

The name of the lake and its valley is pronounced as in was, not with a hard a. The lake is named "Wast Water" on Ordnance Survey maps, but the spelling "Wastwater" is used with roughly equal frequency, including by its owner, the National Trust, along with the Cumbria Tourist Board, and theLake District National Park Authority.