I believe this is a juvenile. It was one of several in a family of Harlequin Ducks (Histrionicus histrionicus) deftly navigating white water on the Yellowstone River. It could have been one of a summer flock of females. But there seemed to be one that stood watch over the others as they swam in the current. That appeared like motherly oversight to me. And the more whitish coloring across the face above the bill on the bird in the image seems distinct from the pattern on adult females I've seen in field guides.
Like Dippers, they feed on aquatic insects by diving in fast moving, clear and clean streams. They may also feed on small fish.
Northwestern Wyoming is the southeastern-most tip of their breeding range.
Location: Yellowstone National Park, WY