Than thank you BobbitWorm45, I must admit that I have seen Lesser Scaup only in captivity before. So I hope that you can write something more, so I can learn.
I know, the iriscent colour of the head is variable in diving Ducks. Bu while I have seen some Tufted Ducks with a green sheen to the head, Greater Scaup have either green (often) or blue (rare) sheen there. I think violett must be within variation for Greater Scaup, but thats the colour for Lesser Scaup according to literature.
While an ID-book Lesser Scaup has a more peaked crown, I still see a slight peak (yes, just within variation for Greater Scaup).
The flanks are burned out, because of overexposure to this area, but with a little bit of guessing, I see some fine barring there, which is also better for Lesser.
BUT: as said, I like to learn here, and would appreciate more comments. Because, when looking in an european ID-book, I got the impression that the Scaup/Lesser Scaup problem is an easy one, but after reading and learning from some threads here, I realized that this is sometimes not so straightforward.