The long sloping shape of the bills of these ducks in the waters off of Occoquan Bay National Wildlife Refuge indicated to me that they are Canvasbacks ducks (Aythya valisineria). Most of the time I rely primarily on colors when trying to identify birds, but during the non-breeding season, many ducks share the same subdued colors, especially when viewed from a distance. This was a rare case when a single distinctive characteristic—in this case the bill—was enough for me to identify the birds with a reasonable degree of confidence.
According to Wikipedia, the duck’s common name is based on early European inhabitants of North America’s assertion that its back was a canvas-like color. In other languages it is just a white-backed duck; for example in French, morillon à dos blanc, or in Spanish, pato lomo blanco.
© Michael Q. Powell. All rights reserved.
Mike Powell 
The shoveler’s another one where the bill can be an identifier for me; it’s not hard to distinguish. I don’t think I’ve ever seen this one.
I remember the excitement when only the 2nd record for the UK turned up in my home county, that was 1997!
Wow. I am not sure how one showed up there–it is always a mystery how a bird ends up so far away from its normal territory.
Often these vagrants get caught up in fast moving weather systems when migrating. Waterfowl can pose problems due to the possibility of escapees from collections so the record committees have to take a very careful approach.
Have a great new year Mike!
Thanks for the info about these one-off sightings. I am much more of a photographer than a birder. Best wishes for a wonderful new year for you too.