Federico Crimi has identified the subject of this sketch as the bridge at Crevoladossola, a town near the Swiss/Italian border where the River Diveria meets the River Toce.1 Part of the composition spills over onto the opposite sheet of the double-page spread, see folio 83 (D14304; Turner Bequest CLXXIV 82).
Turner passed through Crevoladossola during a diversionary expedition from the Italian Lakes to the Simplon Pass in the Alps, and the road into the mountains took him directly through the town where he was impressed by the newly built bridge erected by Napoleon at the beginning of the nineteenth century in order to improve access to the mountains. An impressive feat of engineering, the tall crossing was constructed using wooden supports spanning a central pier (today these have been replaced by stone arches). Further studies can be found on folios 7 verso and 84 (D14157 and D14306; Turner Bequest CLXXIV 6a and 83), as well as in the Passage of the Simplon sketchbook (see for example a similar view, Tate D16941; Turner Bequest CXCIV 27a).
For a general discussion of Turner’s journey to the Simplon Pass see the introduction to the sketchbook, and also the introduction to the Passage of the Simplon sketchbook (Tate, Turner Bequest CXCIV).
Crimi 2006, p.185 note 13.