It is not easy to differentiate between the female American Black Duck and the female Mallard. There is so much hybridization among ducks that it is difficult to tell if a bird is a Black Duck, a Mallard or a hybrid. I am guessing the female pictured here is a Mallard because of the amount of orange on the beak, only a guess. If I had paid attention to the sound, I should have been able to identify them on the spot. The Black Duck says "Quack, Quack" and the Mallard says "Kwek, Kwek, Kwek." That may be the best clue to nail the ID.
I venture a guess that this female is an American Black Duck based on the greenish colored bill and the predominately gray face. Now, all of this begs the question, "How long have I been birding and still can't tell the difference between two of the most common species around?"