We watched this agile denizen of cold rushing Andean streams dive, swim close, and eventually haul out under a bridge where I was able to get these images. The large red feet with indented black webs are likely an adaptation to stand securely on steep slippery rocks as seen here. This sole member of the genus Merganetta is unique in having wing spurs, the function of which is unknown. This is a female of the nominate race found in the Andes through most of Chile. The gray color on the head extends to the side of the neck in females of this race. Digiscoped with Panasonic DMC-LX5 | Nikon FieldScope III | 30XWA | hand-held (no adapter).
References:
Carboneras, C. & Kirwan, G.M. (2018). Torrent Duck (Merganetta armata). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/52858 on 22 April 2018).
Fjeldsa, J. and Krabbe, N. (1990) Birds of the High Andes. Zoological Mus. University of Copenhagen & Svendborg.
Madge, S. & Burn, H. (1988) Waterfowl. An identification guide to the ducks, geese and swans of the world. Houghton Mufflin, Boston.
Todd, F. (1979). Waterfowl. Ducks, Geese & Swans of the World. Sea World, San Diego.
Torrent Duck (Merganetta armata), In Neotropical Birds Online (, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. retrieved from Neotropical Birds Online: https://neotropical.birds.cornell.edu/Species-Account/nb/species/torduc1