Iceland Gull (adult, at Thayer's end of spectrum but possibly intergrade with Kumlien's) with Herring and Ring-billed gulls.

See additional comments on first image.

Slightly extended wing shows mostly white under outermost primaries with low-contrast white mirrors on P9 and P10. Folded primaries above in this image seem lighter than usual for pure Thayer's. Note also bill size and dark iris.