In total, from source to the westernmost part of Duddon Sands, its length is 43 kilometres (27 mi).

From its source the Duddon falls rapidly over a distance of two miles (3.2 km) to Cockley Beck at the head of Dunnerdale.

NOT envying shades which haply yet may throw A grateful coolness round that rocky spring, Bandusia, once responsive to the string Of the Horatian lyre with babbling flow; Careless of flowers that in perennial blow Round the moist marge of Persian fountains cling; Heedless of Alpine torrents thundering Through icy portals radiant as heaven’s bow; I seek the birth-place of a native Stream.— All hail, ye mountains!

From Birks Bridge, through Troutal Gorge to Seathwaite there are some difficult canoeing sections graded at 5 requiring higher water levels to navigate.

Consequently, most of the visitors are day trippers who live locally, or who travel from elsewhere in northern England: Hunter Davies considered its unfrequented nature one of its great attractions.