Are you talking the lake or the Ark? I have never fished the lake but the trout habitat project and the stocking makes the Ark below Pueblo pretty productive, nymphing and streamers are the way to go in the fall/winter. For the lake I would try bunny leeches, wooley buggers, and streamers.
There's trout in the river above and below the res. The lake itself is pretty hard to fly fish unless the wiper action is excellent, then its really easy to get a wiper on the fly, but thats the exception it doesn't happen very offen. During the winter months the trout action really picks and during a calm day many can be seen slurping midges off the top.