In the heart of winter and into the early spring fly fishermen on the open sections of Boulder Creek are very likely to encounter actively feeding fish on the surface taking exceedingly small mayflies or midges. I've found that comparadun style flies can be particularily effective. I'll tie them in a variety of body colors (primarily olive and tan) using micro fine dubbing from Hareline and synthetic wings and tails (split w/microfibbits or emerger style to imitate a trailing shuck) using a variety of materials. A very effective wing (natural flotation) is snowshoe rabbit.
I recall a time in March last year while standing in one particular run near the library I caught and released 30 fish in in 90 minutes on the fly pictured below.