First located in the flock at very close range around 3:20pm. I first noticed it as it faced away from us by its darker mantle compared to ring-bills and initially called "lesser black-backed" to the other 5 observers present; however when the bird showed its face, its dark iris and bill pattern were immediately obvious and diagnostic for California. Bird was slightly larger than Ring-bills, smaller than a Herring. Mantle obviously a shade or two darker than ring-bills Head and nape heavily marked with brown smudging, iris completely dark throughout, round headed imparting a somewhat delicate look. Bill larger than Ring-billed but not as strong as Herring, yellow/green with a complete black ring and, and red/orange spot on lower mandible behind the ring; some reddish also in front of the ring on lower mandible; tip of bill yellow/green. Pale yellow/green legs. Rather long-winged in aspect, with obvious apical spots on folded primaries. Extended wing, upperside: extensive black area at the end of the outer primaries, much more extensive than Ring-billed or Herring, large mirror on P10, substantial white trailing edge to secondaries. Extended wing: underside: same extensive black on outer primaries, distinctively gray bases to primaries and secondaries, much more contrasted than in Herring and Ring-billed. Tail completely white. All features diagnostic for basic plumage adult California Gull. The bird bathed extensively shortly after I first saw it in the flock on the grass. Shortly thereafter it roosted on top of the green bathroom buildings with Ring-bills until about 4pm. At that point it retreated to the water off the day use area where it remained until about 4:10pm when at last it flew off to the main part of the lake to roost. With Michael David I searched the lake for the next hour or so. By dusk nearly 500 gulls had collected on the water off the north shore opposite the Bear Run launch. We may have had the bird in the flock but it was very distant and dark at that point.