We see this all the time on some of our clear creeks.

Here is my take on the events you witnessed. The bright fish you saw was a hen, and that "swimming side ways" gig was her fanning a redd. The dark one was the male. He's all colored up, because the steelhead chicks dig it. Kind of neat to watch.

My buddy has a tiny creek on his property in Northern Minnesota. On a good year, there are about three pairs of fish that come in to spawn. We've watched them do this tons of times. Get's even more fun when there are two males and a hen. Yikes, now I'm sounding like a pervert.
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