ID help

Caught this gull the other day South of Lock& Dam #19 on Mississsippi River. Have not come up with a ID yet. I think it may be an imm. Great Black-backed Gull because the head is larger than an imm. Herring Gull would be. It is slightly larger than our usual Ring Billed gulls.

Among sub-adult gulls with solid black bills, only the juvenile American Herring Gull is this brown. The white margins on the secondaries and tertials also suggests juvie Herring to me, maybe molting to first year plumage. Unless the lower Mississippi Valley attracts vagrants I don't know.

Thanks Jim.