This Sunday before the rains hit, I was able to observe a dark, dabbling duck associating with a female Mallard in Pool 10 nearer to the levee across from the old Eagle's nest.
My initial though was Mottled Duck as I saw an olive-colored bill with a small black spot on at the base of the bill. When the ducks flew to the west side of Pool 10, I saw a very fine white bar on the trailing edge of the speculum. I have uploaded some images to Flickr for viewing.
There is some question that this might be a hybrid of sorts as all the field marks don't seem to align for either American Black Duck and/or Mottled Duck.