Kindly identified by Norman Penny (California Academy of Sciences).
I have the picture turned upright on my machine, but it uploads to iNat sideways. Is there any way I can fix this?
On the ice just south of the airport, along with the large flock of Ring-billeds and many more Herring Gulls than usual; the closest point from which we could view it was the Indigo Landing restaurant area - still a long way away. Stood out very clearly despite the distance, though, as a very large, all-white gull. Much larger (say, half as large again) as adjacent Ring-billeds; appeared about the same size as a nearby imm Great Black-backed. Both body and head shape seemed elongated; certainly not rounded. We took some record photos (through bins!) which show both head shape and size comparison to nearby Ring-billeds. One caveat: one of the photos seems to show slight dark wingtips (we didn't notice these while we were watching the bird itself) - possibly some kind of Glaucous x Herring hybrid? It took off and flew off down the Potomac after we'd watched it for about five minutes - appeared very white in flight (again, we didn't notice any dark on the wingtips when it was flying), and again, very large.
Thanks to Torsten Dikow and Thomas Pape for the identification.