Michael Tat called from the Warden's Watch to let me know about a likely Canvasback.

Although the duck never lifted its head to reveal its bill, it otherwise appeared to be a Canvasback, down to its reddish head.

The duck was still pretty far away, even for a 400mm lens, and light was fading so the head in the photo is not as reddish as it appeared but ...

More input would be greatly appreciated. (Thanks, Michael!)