Yes, it is not unlikely that you caught a rainbow, as a steelhead would be quite silvery if it were fresh from the salt, but I don't think that there are any that enter the lake. At least not enough that you might catch one.

You didn't say why you doubt it to be a bow? Is its coloration wrong, or did you not know that large trout existed in the lake? If you give more details, I might be able to be more helpful.
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Give a man a fish, and you'll feed him for a day; give him a religion, and he'll starve to death while praying for a fish.