There was one male and one female BARROW'S GOLDENEYE with a flock of COMMON GOLDENEYES along the north edge of Bouldin Island, San Joaquin County. This is my first winter in the Central Valley and am not really sure how notable this is for the Delta area.
Here is the male:
Although this location isn't publicly accessible, one might keep an eye out for nearby goldeneye flocks. Here is a map of where the flock was:
View BAGO in a larger map
There was also an adult nonbreeding MEW GULL at the same spot:
Other birds with this flock included many AMERICAN COOTS, a couple of RUDDY DUCKS, and a lone LESSER SCAUP.