Toss anything that floats and is a reasonable sized immitation to whatever you might see in the air. I know, sounds generally unhelpful - but as you've undoubtedly seen in some other threads, there have been fish of all sizes cause on something as simple as a royal wuff. So, for the sake of maintenance I would pick something that'd hold the floatant well for probing. Just concentrate on a drag free drift. If you you see any big fish that are getting shy, then switch it up to a smaller sparse fly.
If nothing is happening on the surface, then give a red or amber copper john a shot. I've always had decent luck on those in the Sno systems. Otherwise - do my favorite activity and start some seining to get an idea of what sized nymphs you ought to be chucking.