Keswick lies in north-western England, in the heart of the northern Lake District. The town is south-west of Carlisle, northwest of Windermere and south-east of Cockermouth. Derwentwater, the lake to the south-west of the town, is approximately long by wide and is some deep. It contains several islands, including Derwent Isle, Lord's Island, Rampsholme Island and St Herbert's Island, the largest. Derwent Isle is the only island on the lake that is inhabited; it is run by the National Trust and open to visitors five days a year. The land between Keswick and the lake consists mainly of fields and areas of woodland, including Isthmus Wood, Cockshot Wood, Castlehead Wood and Horseclose and Great Wood, further to the south. The River Derwent flows from Derwentwater to Bassenthwaite, the most northerly of the major Cumbrian lakes. The Derwent and its tributary the Greta, which flows through Keswick, meet to the east of Portinscale. The source of the Greta is near Threlkeld, at the confluence of the River Glenderamackin and St John's Beck.