This is a declining species due to competition for its invertebrate food from introduced trout, pollution, forest destruction, and damming of mountain rivers for hydroelectric schemes.

Males of the southern nominate subspecies M. a. armata, the Chilean torrent duck, have grey back and blackish underparts, with a chestnut belly.

Males of the slightly smaller northern subspecies, the Colombian torrent duck, M. a. colombiana, are paler underneath, with streaked grey-brown underparts.

Males of a third subspecies, the Peruvian torrent duck, M. a. leucogenis, are intermediate but very variable in plumage; some have entirely black underparts (turneri morph).

The Peruvian torrent duck is sometimes split into 4 subspecies (leucogenis, turneri, garleppi and berlepschi), but these are more likely simply color variations, as they are not limited to distinct areas.