I've had the best luck on rainbow clousers (clousers tied to look like small rainbow trout) and smaller wooly buggers in the higher-elevation streams. This time of year they are low and clear and bigger, flash streamers, at times, can be too much fly. I've never fished Rock Creek but I think it's fairly small, so you might want to try small to medium sized streamers. If you do make it down to the platte , try lead-eye or conehead wooly buggers, and big ones - size 2 or 4. Brown/yellow or olive body/black tail and hackle are both deadly. This is a very nice time of year to fish southeast Wyoming, but the remaining season can end abrubptly at the higher elevations.

Have fun,

-John