Thurstonfield Lough ("lough" pronounced as "loff") is the largest, species-rich area of open water in the lowlands of north and east Cumbria, England. Within this area it supports some of the best examples of a range of vegetation types: from the submerged aquatics through an extensive fringing marsh to good wet sallow and alder woodland. The Lough lies in the Solway Plain of north Cumbria, 5 km west of the City of Carlisle. It is formed in boulder clay and is thought to be a natural feature although the water levels are now controlled by sluice.