Certainly the best current dry fly option for park streams is to present terrestrial insect patterns. These should include those for spruce moths especially if you intend to fish near forested areas holding spruce and fir trees. These areas include much of the Gallatin and Madison Rivers within the park. Other areas where spruce moth patterns are productive include the forested section between Slough Creek’s first and second meadows above the campground and around the campground, Lamar River Canyon, Duck Creek above its meadow, and Yellowstone River flowing through pine forested areas. Be aware of the thick, stop and go tourist traffic presently on park roads. Minimizes delays because of it by entering as early as possible.
Many park still waters are in the summer doldrums. These seem not to impact Beula Lake where Yellowstone cutts remain active throughout summer. You have to “pay some dues” in the form of a 2.5 mile walk off the Ashton-Flagg Road ( no worries about traffic here!) to get there, but inlet and east shoreline walk-in wade fishing or packing in a flotation device will result in a worthwhile experience. Use speckled dun life cycle patterns, cinnamon caddis adult patterns and your favorite small leech patterns.