At first I thought this must be a goose, practically all white as it seemed from a distance. (See small pic to the left.) As I got closer, I saw it was too small for a goose. But it seemed too bright for a Mallard female. Compare, for example, this lovely Mallard female I admired earlier. So, I took pictures and then went home and checked the web. The Cornell bird lab website came through for me with an informative article that points out that Mallards frequently hybridize with other ducks and that the colors of females, in particular, can be highly variable. This bird did have the characteristic orange and black bill of the Mallard.
And there was another clue, now that I thought about it: she had slipped off the rock where she had been posing when first I saw her and paddled off a few yards into the company of another bird, unmistakably a Mallard male.
Case closed, as far as I’m concerned.