Edit - too many anomalies for 'cachinnans' - the more I look at these photos the more I think it's just a Lesser-black-backed Gull. The dark upper-wing and underwing certainly point strongly towards Lesser Black-back.
Inner primaries look slightly paler than outers but that underwing looks yuk. I didn't have the opportunity for a good flight view as the bird lifted rapidly and disappeared downriver into horrible light. Leeb?