Its construction by damming the Gwash valley near Empingham was completed in 1975. During its construction, it was known as Empingham Reservoir. It flooded six or seven square kilometres of the Gwash valley as well as the side valley at the head of which lies Oakham. Nether Hambleton and most of Middle Hambleton were demolished and their wells were plugged as part of the ground preparation. Their neighbouring village of Upper Hambleton survived, and now sits on the Hambleton Peninsula. The Gwash makes a net input to the lake but its flow downstream is maintained. Most of the stored water is extracted from the River Welland at , between Tinwell and Stamford and from the River Nene upstream from Peterborough, a city which is a major user of the water.