[cobirds] Scoters at Valmont Reservoir

I went looking for the Surf Scoters and White-winged Scoters that have been reported at the Valmont Reservoir Complex. I saw 4 scoters, but I'm wondering how to tell that the females are Surf Scoters, as opposed to White-winged Scoters. I posted the linked photos to the ABA "What's This Bird?" page on Facebook and got one response (so far) saying they thought all 4 are White-winged.

Any thoughts?

https://www.fernlakephotography.com/Unlisted/2021/Scoters/n-dKZkNT/i-SqDbmf2/A

https://www.fernlakephotography.com/Unlisted/2021/Scoters/n-dKZkNT/i-HHPnz3m/A

https://www.fernlakephotography.com/Unlisted/2021/Scoters/n-dKZkNT/i-VCNvhhX/A

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