An odd gull.
Weak bill projection with no expansion at the angle, dull #2 pencil in color, not the banana of some other gulls. The feet are the deep pink, pepto bismol color of the Thayer's/Kumlien's/Iceland complex, not the dusky pink of the other large white-headeds. The aforementioned bill projection, while weak, might not be weak enough to convincingly place it in the category with those gulls. The story is the same for leg length, while short, perhaps not quite short enough to convincingly place it with those gulls. The back is quite light and the primaries are not black. If this were Newfoundland and not Oakland, I would shrug and say that it was a Kumlien's with a smidge more bill projection and just slightly bulkier characteristics than you usually see.
As it is, I'm at a loss and my experience is failing me. It's some sort of opaque hybrid, I guess.
Diving raft of 50-100 birds. I couldn't tell what they were with binos. They looks like grebes in the photos. It was curious because they would all largely dive at once, and bob up together at ~1 minute intervals, which meant that one minute you would be looking at a sizeable dark raft of birds and the next there would be nothing. Anybody know if grebes socialize like this?