Gulls are massing between the piers feeding on small fish; I assume steelhead heading out into the lake, but that's just a guess. It's a great time of year to pick up Laughing or Franklins, and we have a few California Gull records for May (to say nothing of the Glaucous-winged) so this bird drew my eye. Here's a montage of the individual at medium distance:
It circled around and flew past a couple times.
A view of the underside:
I know there's pics of Little Gulls like this in the books, but I'm not sure I've seen this variant illustrated for Ring-billed.