I don't have any Golden Cascade/Rouen crosses, but here's the two drakes our Golden Cascade drake and Cayuga female made together. They are the two black ones towards the middle, with their father, "Handsome Henry," in the pool and their mother is the black one with the white feathers towards the front:
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And again in the middle. The Welsh Harlequin on the right is the duck who actually hatched and raised them. They looked exactly like Cayugas as ducklings and their adult coloring is a browner black than our Cayugas (very shiny but not really iridescent)with a subtle green neck ring and an iridescent Cayuga-colored head. They are the same size as their father, big sturdy boys.
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According to David Holderread, most breeds mated with a GC Drake will make sex-linked ducklings (males will be a different color than females at hatching). He said that with our particular crossing, that all ducklings that looked like Cayugas would be male, which is exactly what happened. You could always try contacting him to find out what the ducklings might look like.
http://www.holderreadfarm.com
I think that your crossing will make large ducks, since both GC and Rouens are sizable. Please promise to post lots of pictures when they hatch!!!!!