("lough" pronounced as "loff") is the largest, species-rich area of open water in the lowlands of north and east Cumbria
. 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
. In addition to its vegetation interest the Lough is relatively undisturbed and supports a notable invertebrate fauna and a good range of waterfowl.
Thurstonfield Lough was used as the header tank for the water courses which run toward the Solway Firth. These streams powered corn mills, with different mills working on different days of the week to maintain the flow of water. The establishment of the Lough and its sluice, which still survives today, provided water to the mills even in times of low rainfall. This water management allowed each settlement down stream to run its own mill.
It Is now also home to the Tranquil Otter Holldiay Business