Harlequin Ducks (Histrionicus histrionicus) breeding on the upper Torrent River of the northern peninsula of Newfoundland were studied from 1993 to 2002. This reach of the watershed was proposed for hydro-electrical development in the 1990s. Harlequin Ducks arrive on the Torrent River as soon as ice-out permits in early to late May. Pairs concentrate in the upper watershed that drains the Long Range Barrens ecoregion. Nesting and brood rearing appear to take place primarily in proximity to spring-pair activity, although movements of more than ten km were observed. Young remain in the upper watershed until fledging in late August to mid-September. Counts of Harlequin Ducks breeding on the upper Torrent River increased throughout the 1990s, and the estimated population growth rate for the Torrent River birds was very similar to the growth rate for birds wintering at Cape St. Mary's, Newfoundland (1.14 vs. 1.13) during the same period. Paired females spent ∼ 40% of the day in feeding activities whereas paired males allocated less time to feeding (∼19%) and more time to alert or vigilant behaviors while females fed. Birds rested for 35-40% of the day. Harlequin Ducks exploited rapids, riffles and runs, and were especially associated with the boulder-strewn inlets and outlets of ponds. The large quantities of submerged and semi-submerged angular boulders provide increased surface area for attachments of larval insects, and movement of water through boulders assures high rates of oxygenation important to filter-feeding insects. Broods selected areas with flow rates of 1.27 ± 0.24 m/s. The family Chironomidae contained the most taxa and was the most numerous invertebrate group found in the Torrent River. Chironomidae may comprise the most important food items for pre-nesting Harlequin Ducks when considering volume of prey types consumed. A high rate of brood production in 1997 and 1998, compared to adjacent watersheds, suggests the possibility that the Torrent River system may behave as a source population for the general region of northern Newfoundland.