An amazing duck of fast-flowing Andean rivers and streams. Usually seen as singles or pairs resting on rocks in rushing water and not associating with other ducks. Feeds by diving in churning water and pools and is somehow able to swim up-current with no apparent exertion. Sexes look quite different, but both have a bright red bill and striped back; male has a white head with black lines, while female has a gray head and bright rusty underparts. Body pattern of male varies regionally from black overall to striped with whitish or rusty.