The Duddon is a river of north-west England. It rises at a point 1,289 feet (393 m) above sea level near the Three Shire Stone at the highest point of Wrynose Pass (grid reference NY277028). The river descends to the sea over a course of about 15 miles (24 km) before entering the Irish Sea at the Duddon Sands. In total, from source to the westernmost part of Duddon Sands, its length is 43 kilometres (27 mi). For its entire length the Duddon forms the boundary between the historic counties of Lancashire and Cumberland and since local government re-organisation in 1974 the Duddon has been in the ceremonial county of Cumbria.
The catchment area of the River Duddon includes a substantial part of the south-western Lake District fells, including the eastern slopes of Corney Fell, Ulpha Fell and Harter Fell; the southern slopes of the mountains around the head of Langdale, and the western slopes of Dunnerdale and Seathwaite Fells.