Postscript: Second photo shows size since the bill isn't clear in the first. Under tail markings enough to eliminate other candidates
Sherman Island.
This species is feral/escaped in this part of the county.
Sherman Island.
I was hoping that this lone specimen would be in bloom, but, being beside a trail, it had been severely pruned back by humans and was showing only new spring foliage.
Sherman Island.
Sherman Island.
I saw 23 of these at this site and not a single Pond Slider!
Sherman Island.
A small flock of yellow-billed subspecies winters at this site, the only spot in the county where they are reliable.
Sherman Island.
Male.
Sacramento River at Sherman Island.
Female.
Sherman Island.
These were the closest of a flock containing more than a thousand individuals.
Sherman Island.
This observation pertains to the two centermost individuals standing erect. They were in a huge flock than was over 90% Snow Geese.
Sherman Island.
Male, at center.
San Joaquin River at Sherman Island.
Specifying the closest male duck, at left center.
San Joaquin River at Sherman Island.
There were over 50 of these here, the largest flock I have seen away from Mather Lake. Under half show in the second photo.
Sacramento River across from Rio Vista.
Photo taken from main Auto Tour road at Yolo Bypass Wildlife Area.