This is a Hooded Merganser, and a good lesson in not relying on a single field mark.
Among other things, the long skinny bill makes this a merganser.
Although Hoodies don't normally show a whitish throat, and indeed Sibley's doesnt show one, juvenile birds will. For example, see here:
http://www.birdfellow.com/birds/hooded- ... tificationThere are a few other features that point towards Hooded here. They will sometime raise their spiky tail like in this picture, I don't think I've ever seen a Common or Red-breasted do this. The dark back and paler sides, and the brownish colour overall indicate Hooded, Common and Red-breasted have smoother, grey bodies. Common and Red-breasted will also generally show a much more obviously rufous head.
Two other clues aren't visible in this photo. Size is very important - Common Merganser is one of our largest ducks, very heavyset and larger than a Mallard. Red-breasted is about the size of a Mallard but slimmer. Hooded is much smaller - Goldeneye or Scaup sized but slimmer.
Habitat is also important - in general Hooded Merganser is by far the most common species on sewage lagoons. Commons do show up. I don't know if I've ever seen a Red-breasted on a sewage lagoon.